I was in a woodland mansion on the bottom floor, and I heard the mobs above me following me around from above as if they knew I was below, despite there being a floor in between us, and generally blocks will block a mobs detection radius. The moment I approach the stairs, they all storm down them and I am attacked by an angry horde of Vindicators and Zombies, and died.

I've never seen this happen before, only this one time with a woodland mansion. It may happen with any kind of floor, but this particular incident, they were able to see me through the floor of a mansion. This makes absolutely no sense. Why should they be able to track me through the mansion floors? When they can't track me through walls?