I thought possibly related to #2910, but I'm using the latest version (r2.4-304).

This is how it looks before being saved.

This is how it looks after being loaded (and after breaking a block to show).

This happens in all directions; they're flipped from the direction they should be. The structure is not being rotated, the back of the room always faces East.

latest.log (pastebin)
debug.log (file)

Weird. I just inverted the rotation in the code, maybe it helps.

Works perfectly now