It is very bad that centralized exchanges can do this type of stuff.
Bitcoin fungibility is important to be preserved and these type of actions from centralized exchanges is what makes it harder for perfectly fair traders to not discriminate against normal and perfectly valid bitcoins.
Bisq will never be able to hold your funds hostage like this, due to it’s decentralized nature. It is worrying that immoral actions by centralized exchanges can make someone consider actually not using the only type of exchange that can not hurt you this same way.
It is ironic. Centralized exchanges bully their users to stay away from decentralized exchanges that can not bully them.
It is perfectly possible that this is the exact same reason why they are flagging your coins in the first place. There multisig transactions might make them figure out that you are a Bisq user, using the only fully open source decentralized exchange that they can not buyout or get government to close it down for them.
This might me against certain antitrust laws if they are dealing with their competition like this. Maybe even it would be a good idea to point this out to them, I don’t know. Maybe a lawsuit will worry them enough to release your funds.
PS: We should wait and see if some other Bisq users experience the same type of issues with centralized exchanges. I don’t use centralized exchanges myself, but I never had problems before with vendors accepting my BTC for payment.