It was inflation of an amount you can't tell, but you can detect the transactions that exploited the bug. If you want to call that "hidden inflation" I understand why. It's completely distinct from a real hidden inflation bug where you actually cannot detect a valid transaction from a malicious one. Monero has never had such a bug patched in its history. Just like in Bitcoin's overflow bug, you could detect the transactions exploiting the bug which allowed Bitcoin to roll back and erase that transaction from history. That is also the case here, however this is a sidechain and the Bitcoin was already stolen. Bitcoin's overflow bug would have led to this exact same consequence on Liquid.

how come maintainers not catch this before ? or keep open bounty for such things