I'm not entirely sure what vanilla would do here, but it's quite unsatisfying.

This is technically not floating but there is no block in front of the portal which IIRC is prevented by vanilla selection.

The first one is normal, I intentionally made portal sometimes generate above lava lake. But the second one is a bug.

The second one will probably not appear in 0.21

And it's still possible to generate the portal above lava lake