If a Mongol player packs up a building, the old footprint remains un-constructible until Mongol places the building in the new location. Appears as a gray hashed square where the building stood when you try to construct. This was noticed specifically when a Mongol town center packed up and while attempting to construct a ram with spearmen.
Thanks for reporting @Heftydogg! I’m not seeing this in the live or PUP builds, but I’ll keep an eye on this thread to see if it crops up again.