I've found out that floating there's some blocks that won't print. for example, floating slabs won't print at all if not connected to a full block.

+1, have to place temporary blocks all over the place

This is because as Litematica is mainly used for SMPs and other things like that it would be more like exploiting placing a block without support.

+1

It has nothing to do with it "mainly being used for SMPs." No idea where you'd even pull that fact from - Litematica is used for a number of purposes. It's entirely down to the "placeInAir" functionality being removed after the 3.0 rewrite. This is the #1 issue the mod is facing right now.