I have 5 Villagers in the ground in the location where an Ocean Monument is. At night, I've been having Zombie Sieges spawn in despite the rule where hostile mobs cannot spawn within the bounding box of the Monument.
What Happened:
Zombie Sieges spawned in the Ocean Monument when they shouldn't be able to.
What Should of Happened:
Zombie Sieges shouldn't of spawned at all
Steps To Reproduce:
1. Find a Monument
2. Build walls around it and clear the blocks/water down to the bottom-most layer of the Monument
3. Place a few Villagers with beds and work stations, then wait for night
Please check if this is still an issue for you in 1.16 Release Candidate 1 or later.