Weird green rectangle - default client

[Mr_Bober]

I play on the standalone default client (not Steam).

Today I logged in and saw this weird large green area.
Ortho view:

Bad view:


It wasn't there yesterday.
It intersects with our personal claim, so it's not a claim. Showing / hiding claims (both personal and vclaim) doesn't have any effect on it.
I can interact freely with the items that are in the intersected area. I can also put building signs.
Logging out and back in doesn't make it disappear. Neither does going outside the cave and back in.

As far as I can tell, the other cave it intersects has no entrances, so I doubt it's something player-made.

[Mr_Bober]

I left the area for a while, teleported back, and now the green rectangle is outside instead of inside