Have you check if the origin of an entity is not at this position, outside your level? This is the most common cause for this type of trouble.

Everything is in place.

Last I used source, displacements do not seal the map. looks like you have foliage looking models in your level, so are you using terrain to seal the bottom?

I've figured it out already. The map had no leaks, but for some reason if I turned off the skybox visgroup the game thought there is no skybox at all.

引用自 Dorotpole：

I've figured it out already. The map had no leaks, but for some reason if I turned off the skybox visgroup the game thought there is no skybox at all.

Turning a visgroup off effectively removes every object within it from the map. They're "hidden" in the editor, but they're not there at all in-game.