Hello. I'm posting on both Weather2 mod's and Better Portals mod's page. This is mainly a QoL bug. When looking at the portal being able to see into the nether. The weather above and any rain falling stops being visible. Looking away from the portal resumes the weather effects. I was about 20-30 blocks from the portal but seems to extend as long as the portal is within render distance. I'm not sure which mod is causing the bug so that's why I'm posting on both. Here is a gif of the bug https://i.imgur.com/BZXapxt.mp4. When I'm turning my head right is when I'm turning towards the portal.

I also attached my current config for both mods and optifine settings. forge version 1.12.2-14.23.5.2836

weather2 mod current config.zip

Please report this to the weather2 mod as well. Chances are they simply didn't expect multiple worlds to be loaded at the same time (and they should fix that). Or, considering it only happens when you look at the portal, their rendering code doesn't work properly when alternating between different worlds for each frame.
Until they send you back here, I'm going to assume it's one of the above.

I have but the author doesn't seem to be very active. My current work around is I have moved the portal just passed the render distance with a rail system to get to it.