We are also experiencing the same critical issue on our projects with an apparently random panel of iPads on various versions of iPad OS 13 and 14!
UIImage(named: "***") returns nil on those devices for assets with idioms iPhone and iPad while assets set as universal work properly. No such issue with other major versions of iPad OS (such as 12 and 15), nor on iPhones.
Observed at least with XCode 13.2 and 13.3.
The issue first appeared on 2022, March 7.