[FWIW, yes I did delete the library entries per the instructions]
After digging deeper, it's a *lot* more bizarre! Looking carefully, I discovered that there were 3 movies that were imported correctly (out of ~1000 total)! So, I:
1) Set up a test directory with 3 movies in it (one of the three "special" movies, and two other random ones
2) Totally wiped the Library ("Clear Library....")
3) Deleted all MediaCenter files from the Movie directories (e.g. Sidecar, jpg). Verified that all three movies had identical ownership and permissions on all files.
4) Performed a Single Folder Import on the test directory
Result was that the special Movie ifo was imported as Video, the other two were imported as Documents.
? ? ?
Setup:
Kubuntu 22.04, MediaCenter 29.0.41 installed via installJRMC script. Movies were ripped using AnyDVD, and all three movies imported and played without issue on Windows MC.