For me I see it as fair, I am not saying that the casino is right and OP is wrong or vice versa, I am speaking from a neutral point of view, since the capital was returned, the OP did not lose anything, he just lost his winnings which he thought he deserved (which is the point of contention ).
In some casinos, the player's money is fully reserved and his account is frozen if there is a violation of the terms of use (according to the casino's belief, of course), while here only profits were reserved, so I said it was fair.
I don't think about loss only after Betcoin.ag refunding with deposit fund only without give all winning fund from betting, how fair is it when some one loss their betting sport there are never have guarantee with their money can back, but when OP get winning from betting sport why account suddenly lock and costumer service refund with amount of deposit only without calculated with how much fund in Betcoin.ag account.
I don't think its fair what did by Betcoin.ag, have clearly which term of service break out by OP and I don't think with huge reputable casino platform have weakness or bug can give profitable for gambler.