I ran into a case where I had to import in my model at the lowest subdivision level as an obj, it had polygroups and would import them in at the lowest sub d level. Once I stepped up in sub division levels the poplygroups were destroyed and there was only one polygroup for the entire model. This happened several times.
To fix this I used some of the poly grouping features, such as group by “UV Groups.” Then I imported in my lowest sub d level and everything worked.
No resolution on this yet. Has anyone else seen this issue?