I first encountered this problem in 1.16.100, or perhaps it goes back as far as 1.16.20.

The Problem:
When it rains, villagers initially get under a roof as they're supposed to, but after a short time there, sometimes they walk back out into the rain. Then they suddenly realize it's raining again and run back to their shelter. This repeats for as long as they would be awake and working, or until it stops raining, or until it's time to sleep.

The bug affects all kinds of villagers, regardless of profession. There is an exception for clerics, in that due to MCPE-110777 they sometimes stay on top of their brewing stand without moving, even in the rain.

In experiments I learned that this only happens if the "roof" they're under consists of nonsolid blocks (e.g. stairs or slabs). If they have solid blocks above them, they simply stand inside as expected.

I have attached a world that demonstrates this. It's easier than writing out repro steps.