Fill gaps generates strange vertical wall

[mrvanes]

The attached stl generates a strange vertical wall when Fill gaps is enabled on otherwise normally behaving ABS profile. I've tried repairing and cleaning up the stl, but that doesn't seem to help.

KeyChain_mfta.tar.gz

[supermerill]

IMO that's due to the badly tested PR #4472
PR #4790 should resolve that.