Mobs getting into a composter or cauldron that has air beneath it gets stuck indefinetly,
As soon as a block is placed directly below it, they realize they can jump out of it and will do so when pathing.

Note: If the mob is spawned inside the composter/cauldron, with a spawnegg for example, it will jump out of it regardless if there is air beneath it or not, but if its pushed into it, it will stay.

See attached screenshots.
(sorry for the dark images, its the easiest way to illustrate the villager is unable to go to bed untill a block is placed beneath the composter.)