Is it a mod compatibility issue?

Yes
If it's a mod compatibility issue, please provide the link(s) to the incompatible mod(s)

https://modrinth.com/mod/better-clouds/version/1.7.4+1.21-fabric-r
https://modrinth.com/mod/immersiveportals/version/v6.0.3-mc1.21.1

Minecraft version

1.21.1

The clouds do not render properly and this weird visual bug happens when a portal shows the overworld

The clouds do not render at all when you look into the overworld from a portal in the nether

When you look from the over world trough a nether portal that lets you look trough another portal in the nether it will render the nether and stop rendering the overworld.

Turning on shaders partially fixes the issue but causes the clouds to flicker when moving arround

Another example but with regular portals, clouds don't render trough them and they flicker/jitter/shake quite a lot because the other portal is facing the sky. (this happens in all cases where you can see a portal trough the sky)

Betterclouds version is 1.7.4
Iportals version is 6.0.3
Minecraft 1.21.1

I forgot to add what the last portal setup looks like without shaders enabled so here it is.