OIC. Yes, there's something buggy going on there. Before, I was sorting by year then grouping by genre, or the reverse, and it would work fine. Same with rating and genre, I believe. Now I see it can "forget" the sort setting and apply some other sort when grouping is added or removed—while still showing the same field selected in the sort menu.
What it seems to be doing is "remembering" the sort order separately for Plain View and Tree View. So if you switch to Tree View (or apply a grouping) it will be sorted the same way it was when you last did that, rather than how is currently set for Plain View. Perhaps that's by design, but it's very confusing. Especially when the sort menu still indicates it's still sorted the way it was in the previous view. Grouping and sorting should simply be independent of one another.