When creating a new subsurface layer with "maraxis" planet mod installed, it creates an empty void with occasional Maraxis fish floating around, also it is impossible to leave the void once entered.

Does this occur when creating underground on Maraxis or also on nauvis?

I'm getting the same issue. It occurs when creating an underground on Nauvis.

Just flew around, it happens on all planets, Maraxis included.

Please disable Maraxis mod and check if it still occurs on all planets

just tested with all my mods updated. Maraxis is, indeed, source if the issue, on both new and already existing saves.

Although, it seems to occur only if i add Maraxis on save with existing subsurface or create a new save with both mods enabled, but if i add Subsurface on save with existing Maraxis (where Subsurf wasnt present before) it works fine on all planets.

In Maraxix mod, there is a file prototypes\planet\trench-noise-expressions.lua. Change line 159 to -20000 and test.
But I still don't know why it works after adding Subsurfaces...
And try other mods that create surfaces, e.g. Factorissimo.
And pls give me the name of the fish that are floating around

i will mention that it works flawlessly with factorissimo 3 its even made by the same dev as maraxis

I made an update. Should fix this.