Technically the floors are built under them. Build some graves, then tell your colonists to build some floors where the graves are, and they will build floors under them. I feel like this should have been reported already, but a search for "floors graves" yielded nothing.
I can't verify right now, but I'm pretty sure that graves can't be built on floors or stone (since you have to be able to "dig up" the ground). Which would make this a bug or an unintended feature.
Can someone check this sequence:
1. Build grave
2. Build floor under it
3. Deconstruct grave
4. Attempt to rebuild grave (I think this will fail due to floor)
I can verify that doesn't work. You can't build graves over the floors whether or not a grave was built there before.
Yeah, I think it's a bug.
Fixed, thanks.