v1.20.62
A week ago this started happening to me on a local world quite regularly, usually for the first load after starting the app. After loading I'm at the spawn with literally nothing. If I immediately leave the game and load the same world again, everything is back to normal.
This doesn't feel to be resolved.
Bedrock v1.20.62
Villagers located in the room 14 blocks below the surface under my house (healed from zombie villagers) make illagers consistently spawn underground. Explanation by GoldenHelmet makes perfect sense. I literally was unable to find them even by switching the copy of the world to Spectator mode and searching for 20 minutes.
Happened right now to me, based on my testing the amount of consumed RAM was proportional to the render distance settings, on 32 chunks it's 14 GB+ in under 2 minutes of gameplay and computer starts swapping heavily.
Bedrock 1.21.51
After some more testing it happens ONLY when loading a really old world (created 3 years ago). This was the setup:
My son on another account started really old world
I connected getting all my RAM lost in a minute
Consistent behavior after multiple restarts - world is unplayable in this condition