Check the attached screenshots - there should be no logical issues to force that 2x2 tile to not be able to snap successfully here. Same goes for 1x1.
Doesn't matter if snapping is one-by-one or continuous (lmb hold).
It also seems to be dependent on the order I put the floor here - different tiles will be un-snappable depending on that order of putting them down.
I can workaround that by either:
- pruning small stone blocks and re-adding them after the floor is added from the bottom
- forcibly mis-aligning the floor by few pixels on the horizontal plane in the snap-off mode, so it gets a new "snap edge".