My guess is they want to give you a bonus with a high rollover.
Most likely. I've never bothered to ask about the terms.
I've calculated the number based on your screenshots myself as well as inquired to them. I guess I'll begin with their "opening" number is not 375,000, rather 330,000, because one of the bet is actually losing before they got cancelled, I guess it's not shown on the images you provided. So, 330,000 USD, 330,258.69 to be precise.
From this number as our opening number, if we're going for half, it'll be 165,129 USD, and you've got 119,750 from the stake returned. Plus extra 5,000 USD bonus that I don't think mentioned anywhere [feel free to confirm or deny]. Using the "opening" number they proposed, halfed as a gesture that both sides willing to meet in the middle, and substracted by what's returned to you:
USD 330,258.69/2 = 165,129
USD 165,129 - 119,750 - 5,000 = USD 40,379
If you're agree to that number?
I don't agree. The bets would've won over 400k+ (about $410,000 I believe). I've already compromised by offering a smaller sum (330k as they calculate it, fine), why would I accept half of that?
As to the $5k, yes, they did give me that as cash, as an apology gesture after they made some accounting errors (which they fixed after I pointed them out). It's still sitting in my account, I've never withdrawn them.
First, abaout the discrepancies between 375,000 and 330,000 [i'll do a rounding here as we all understand the numbers being talked about]. I've inquired to them and they informed me that the losing leg is this bet,

and I didn't take their words at face value and check, of which, it
was a losing bet,

Which means, 46,000 were subjected to be deducted from the "opening" number, and it brings us to 330,000, like the number they proposed.
Now, that established, I'd really appreciate if you're going for an open mind here as a gesture of your good will, as they're also agree to attempt this once more instead of full voiding, as a gesture of good will from their own side. It'll be very nice if both parties honored the wish and words of each other, that they'll truly meet in the middle.
You proposed that they honor the cashout and they propose to honor half of the cashout.
I'll say 40,000 is still a good thing a good thing compared to nothing.
Moving further, I'll lay it out in the open and publicly admit there might be an oversight from me. I raked upon their ToS to find clause that justifies them to cancel your multi bets, and they
do don't have a clause in their tos where they can cancel multi-bets unilaterally, the ToS they have about multi-legged bets was talking about a cancellation due to match didn't finish. I've read them word by word, letter by letter, all in their
point 4 of "Placing a Stake".
However, apparently, they do have a clause on point 6, that say,

and I regret to inform you that based from the information they provided, not only they have that right, from 6.1., but also further emphasized with an enough reason to void [so the call was not done baselessly or out of grudge or other malicious intent] as per 6.4.3., unfair practice, which was flagged by their provider.
They have their eyes on you for so long for the technique you're utilizing, and I'll do the courtesy of not divulging to the public of your betting strategy that they and their provider deemed as a violation. But given they've set their eyes on you for so long, that the provider flag you as a very high risk bettor, I believe you've repeat this more than plenty and fully know what you're doing.
Personally, upon hearing their rebuttal, I think that's a clever strategy and there's nothing wrong about it. But unfortunately, upon doing my own DD of raking their ToS amongst several other things, it does match the point they prohibit on 6.4.3.
Bottomline: they do have the right to cancel your bet. Unilaterally.
Now, I'll emphasize this [suppose it is not clear enough] I couldn't care less of the outcome of this case. You refused to meet them in the middle and choose to escalate by flag them, maybe some DT will support the flag, depending on how you craft your wordings for the flag. Other DT will not though, upon this revelation of possible breach of ToS.
Still, if you somehow managed to activate the flag and their reputation got tarnished, my world will not end. I'll just wake up tomorrow with same brew of coffee, sitting in my chair and reading work related IRL emails and the forum. That much the outcome of this case affect me. Whether you come out with some funds in your pocket or they got flagged, it affect me at the barest.
However, I do really hope that the outcome is you with more funds. As I think it is very much favorable, if I were you.
Even further, suppose someone who [we're talking imaginative person here] said they have arbitraging cases longer than me [well, I'll take their words at face value as I never arbitrage, I'm simply bridging] suggested "go to ADR, got to AG, got CG, go to their licensor, sue them," I personally think those third party will rule in favor of the casino following their explanation and the evidence they also shared with me and the ToS you breached.
Plus [not posting this to brag, but rather to emphasize that I don't think you can get similar discussion on AG or CG or whatever else others whispered to you], this:

I deliberately cropped it that way to show that it is not me typing it. As you can see, it's on the left side of the chat, not right side, it's their bubbles. And no, I did not violate the privacy I guaranteed to those who reach and share through personal means of communication with me. My oath of silence was not broken, because that is part of their final standing that they'll publish.
To give a beter context and to show you [and public who relentlessly accusing me of always siding with casino] this is what happened for the past one month and few days since I get ahold of their contact:
They come to a verdict, namely voiding your bets. I asked them to reconsider and argued several points. They asked time to discuss and come bac and informed me that unfortunatelly they can't budge, and give an even further explanation of why. I asked them again to reconsider [practically begged them] and wondered if they are willing to meet in the middle with me bridging between you both, to a desired outcome. Then come the third statement.
Three times they made a verdict, and each time I asked them to reconsider.
The snippet above is their final standing. So yes, I don't think you can get this discussion on AG or CG or whatever ADR, as those ADR will very likely close the case and rule in favor of the casino following their explanation and supporting evidence.
And this is where I stand, where I previously ready to drop this case as it bring me too much headache. Yet I am here, with their final offer, so you can at least get something.
40,000 USD is the offer they have in hand. You can take it and we can each go to our separate way, or you can continue your battle and I'll stick to my earlier statement of this remove myself as I don't have any interest in gulping any ergotamine.
p.s.: suppose someone somehow managed to try to twist this is me "thratening" the player or "forcing" him, no. I am not. I am simply laying everything in the open. I couldn't care less what his decision is. He take the offer? Good. I'll convey and get the fund delivered. He refuses the offer and pursue another path? Also good, I'll take my leave and will be waiting for an update of the case from spectator seat. I'll still get my inner peace regardless of the choice.