The bug lets villagers enter beds even though they are separated by gates and solid blocks. The exact requirements for this to happen aren't clear, but I first observed in this villager breeder setup which it disrupted by letting the villagers escape.

To reproduce it, create a wall, dig a 2x1 block hole, add a trapdoor to prevent the villager escaping and put a fence gate on the bottom block. Then place a bed next to it on the wall. In some cases, the villager will be able to get in the bed.

For me the behavior stopped when the gate was removed, but I am not 100% sure this prevents it entirely and encourage others to do further testing. I have attached a video of 5 instances of this bug under slightly differing circumstances in a testing world here (the first three are recorded in 1.21.4, the last two in 1.21.1, but to my knowledge the bug hasn't changed since then): 

https://drive.google.com/file/d/1hLCFx-vQn1D7wNHh_XwwGKKeHcDK1Sm8/view?usp=sharing