[CSGO] BUG: Parts of the level disappearing, what could it be?

Hmm, this looks similar to an issue I'm experiencing. In my experience when something like this happens it has to do with visgroups and some sort of problem with vvis. What is your speed set to for compile time? The setting looks pretty interesting. Hopefully someone else can point right to the problem.

Sometimes it could be because your visleaves are too messy. Are you ok with the leaves concept ? (If no I can try to explain a little or if nomeone has a proper link to the basics :p)

If theyr are too complicated, the engine will have trouble decinding what are the area that need to be drawn. To check how they look, after a compile (just vbsp is enough) in hammer, check map > load portalfile. They will appear in blue.