So I'm not sure if this is a bug or not, but I think it is. I've noticed this for years now and it has been so prevalent for me that I often turn off weather in creative worlds.

If it starts raining/snowing/thundering, I will often try to turn it off with commands, but when I do that, about 5-6 minutes later, rain/thunderstorm will spawn, more often a thunderstorm I think. I'll despawn it, and sure enough, another 5-6 minutes later, another one spawns.

I've noticed this for years now, so I think it goes back a few versions, but I'm not sure where it started exactly.