Drag and drop a new Ultra Dynamic Weather in your scene, Select it in the Outliner and then go to details tab and search for word ray, there you will find this setting.
This happens due to ray tracing not being able to render your rain correctly, turn it ON or OFF.
This should solve your problem, because i know you won’t find anything related to this problem anywhere, happened to me before and searched for it everywhere and then had to try everything through which i found this solution.
If you still don’t find any solution, try the discord server of Ultra Dynamic Sky, you’ll surely get answer there if this setting doesn’t work.
You can get the link to the discord server from their Fab Marketplace.
