Steel Door issue (Thermal Update)

It's a buried object. Seen it happen on one of my doors as well. Have you tried sweeping the tile, or dig out the tile beneath it?

Can Confirm same thing.

You do not need to dig under or something. It behave like normal naturally filled up tile (like it never where digged).

Sometimes it is visible (with fallen sand-like effect), sometimes it is without any effect (it is invisible with cracked 'burried object'-like effect). I couldn't track why 'sometimes' happens yet.

Workaround is to simply dig it again or build something in the same spot.

Edited March 18, 2017 by qb.
clarification, workaround

You can then get rid of the (quite unuseable) material by feeding it to a hatch.