The bug - Minecraft JAVA 1.20.1 Vanilla :

In the iron farms that I make, several times it happens that the iron golems spawn inside the glass blocks, instead of the correct place, which would be the golem killing zone.

This bug according to my experience making golem farms, appears between 1.16 - 1.20.1 in all updates starting from 1.16 where in other worlds in that version towards farms, I don't know if the same thing happens in 1.15 and 1.14 but I guess so.