TL;DR: Screwed up in color painter, object still had miniscule layer of external filament, use the “Bottom” pop-up to ensure precise layer-filament selection.
Had the same issue, ended up being my own carelessness in the color painter. Mistakenly selected the TPU I had set in the manual/external filament slot when painting by layer; realized my mistake in the moment and tried to eyeball the layer line when I replaced it with the correct PLA filament in the AMS. Missed it by .02 mm, which was less than my layer height of .08 mm, but slicer didn’t seem to care about that; .02mm of external filament on the object was enough to make AMS throw a fit. Went through and fixed it using exact measurements for each color via the “Bottom” pop-up, which in turn cleared out the tiny bit of residual TPU painting and everything worked fine.