The case has been going on for 5 months and we still aren't sure the reason for the KYC check. Is the amount of $12k? Other books don't require a mandatory KYC check for $12k. In fact one bet can be $12k. Is it something to do with his bets such as bot betting?
This is the first thing that we should know. Searching for a reason to confiscate money is unacceptable. It's similar to illegal search and seizure.
DabsPoorVersion understand the situation wrongly, the amount being disputed here is not purely from deposit. Granted he deposited to a sum larger than the amount in dispute, but according to OP, inferred, the amount in question [USD 12,000] came from winnings,
The situation OP experiencing right now [a requirement to do KYC] is not purely because he deposited certain sum of fund. It might be related to AML policy, but there also possibility of other violation or perhaps even small misunderstanding.
The way I understand it, that KYC is simply the preliminary requirement by casino to conduct their investigation, which is a quite common practice shared by many casinos. The problem is yet to be announced. OP never knows and Stake doesn't explain it yet.