Firstly, right-click the ground with a stick and see who has claimed the block to make sure that it isn't another player.
If it says "No one has claimed this block", I have a couple of things you could try to fix your claim:
#1 Place a block on the floor to update the claim.
#2 Try resizing the claim above ground first, then do #1.
If none of these work, I would suggest that you create a
bug report. Claims below 0 may have some problems.
Update: I have tested this myself, and I can confirm that you cannot interact with claims below 0. This is a bug.