In some recent-ish versions of Minecraft (1.18?), the fog has been modified from cylindrical to spherical. In some even more recent versions of Sodium (0.5?), it has been updated to match vanilla behavior. That causes the world to stop rendering when flying over at high altitude, because the world is culled behind the spherical fog.

Some people prefered the cylindrical fog (and related, the cylindrical fog culling), but at the moment there is no great option to enable cylindrical fog with Sodium (sodium-extra can disable the fog entirely, and that works properly with sodium fog culling, but that also disable distance fog, not only vertical fog).

Related to CaffeineMC/sodium-fabric#2026, which I think is an overreach for base Sodium, but is on topic for Sodium-extra.