https://youtu.be/xWSEc_d0sh8

Having a mob in a chunk seems to be causing the neighbouring chunks to stay loaded.

World spawn in the example was at x3/z3. The mob at z500 unloaded, but the rest of the chain stayed loaded.

Is likely connected to the fact that entities seem to be processed regardless if they are on the edge of the loaded area or not (formerly known as 'lazy chunks'), and range of mobs scope is larger than just its nearest vicinity.