For example, if they detect multiple accounts somehow, they would flag the withdrawal and ask for KYC before they suspend the account.
I just mentioned one possible reason/criterion. Other possible reasons could flag a withdrawal for manual verification. As for spinly, they approve withdrawal manually regardless of the withdrawal size. Of course, they have some limitations, which is why the withdrawal is manual. But they will change it in the future.
The example you give is very good and appropriate and it is the way to understand how a casino can act in the face of all this, Cases like this have occurred In fact, sometimes it is not even the person's fault, because it may be that a person enters the casino and his uncle, his father, his brother and sister, his wife want to enter the casino to play, and they are all different accounts but with the same public IP That's where things can get complicated. I think that is the point of KYC at that time and it can be determined that it is not an exploit.
You could complete KYC and still get ban by some casinos. This is just too complex to figure out and most time I don't like it when a casino try to detect multiple accounts only through AP address which I know can always look the same if we must judge from that.
Casinos have different tools they can use to detect and suspect multiple accounts on the casinos and I don't opt in to the use of IP addresses. That is an old pattern that is not so effective especially in a school environment where students connect to general WiFi to complete assignments and do other things.