I have also experienced this.

Which I suppose is the correct way of dealing with it. However, the information on the Custom Map remains wrong, because it clearly states that My Pet Dungeon rules are available for that kind of map, when they aren't.

Additionally, the Toybox room is also unavailable in Custom Maps.

Sorry for the delay, here is the save file... Save File and Meta File. If you need anything else, let me know.

I've seen that behavior repeat itself in all Pet Dungeon Levels. The only solution has been, indeed, to drop the Imp flag on the forge I wanted to use to provide the defense parts.