In-Game CSG is unreliable in an online game. There’s really not much that can be said here… even really trivial operations (flat part subtracting another flat part to make a through hole) can appear to fail seemingly at random.
For example, in a studio play solo session the feature works perfectly:
But in an online game, the result of a CSG operation has a very strong tendency to disappear.
The result of the CSG is physically there (you can collide with it like you’d expect to), it’s just that it’s not being rendered at all.
One way that I’ve found of mitigating this is to delay parenting the result of the CSG operation… but that sort of makes the feature not worth it (this delay is on top of the time it takes to do the CSG in the first place) + this isn’t perfect since you can’t tell exactly how long you have to yield until the CSG is “ready to render”.
The game referenced in the clips I posted above is the example linked in the devhub article for In-Game Solid Modeling: