12.1.1 “Part” functionality having issues with Dataset

I made some large datasets of SEM micrograph images and metadata in 12.0 and they have been working no problem for several weeks. but now when I run the notebook in 12.1.1, it seems that it fails to define the Part of the dataset I need. I just reinstalled 12.0 and the problem is still happening now, so maybe I’m just an idiot.

Dataset

Output