Okay so, it seems to be quite inconsistent as to whether or not this occurs, but I seem to have found steps that consistently reproduce it for me - and have confirmed it still happens in V1.25 (the experimental).
If I create a new Marble level, and first select the background, then add two terrain pieces, then an object, and then open "Just Walk" from LPI, it seems to consistently trigger this issue. This is the only sequence of steps, so far, that has managed to reproduce the issue for me - including that opening other Dirt levels doesn't seem to trigger it.
EDIT: Okay, I think I've found the key detail that was missing. There is an additional requirement for this bug to trigger: The level with the background, and the level that you load, must be the same size (or at least the same width - haven't tested differing heights yet). EDIT 2: Yes, it applies to height as well. Expected as much, but always good to test.