UPDATE:
After using "--forceUpgrade --eraseCache" it initially didn't fix the problem.
But a day and a few server restarts later, the problem seems to have gone away.
So I'm guessing eraseCache + a restart and some patience will solve the issue.
Alright, I've tried following the instructions for the "--forceUpgrade --eraseCache" thing.
I've tried it both through the control panel and by downloading the world and doing it in single player.
The problem is still there.
[Edit] yes, I did use the world in 20w17a
I'm experiencing the same problem on my survival server.
It is not just the rain/snow though.
Water won't freeze in the patches where is doesn't snow/rain.
I downloaded the server world and opened it in single player, and the problem still persists.
I can not make it happen when I generate a new world with the same seed.
I find the problem with the weather in multiple different places in the problem world.
I have not been able to figure out what the affected areas have in common.
Is that enough info?
In my experience, the problem seems to be far worse in deep ocean biomes than in most other biomes.