Ours seems to be working correctly now.  As far as I can tell, we had three separate issues that affected different courses:

1. Canvas pulled data through the old SIF connection after we rolled over the school year, but before they had finalized the OneRoster plugin.  When they started pulling data through OneRoster, the sections and enrollments created via the old SIF connection were still there, meaning we had duplicate sections and some outdated enrollment data for kids whose schedules changed after they switched to OneRoster.  I solved this by running a sections export, filtering to only SIF-created sections for this year (they had a much shorter section_id than the OneRoster-created sections), changing their status from "active" to "deleted", then importing to delete those sections.  Canvas Support approved this solution after initially suggesting something that would have cleared out all of our manually-created classes.
2. Presumably trying to fix the broken grade passback, we had some teachers turn off the "automatically sync grades" option, so even after fixing #1, their courses weren't syncing grades.  Turning that back on and manually syncing existing grades did the trick.
3. Some teachers were initially able to sync assignment names, but not grades, then (seeing that grades weren't syncing), deleted the assignment in PowerSchool. This caused an error when that assignment tried to sync over.  This was fixed by resetting integration data for that course, clearing out assignments in PowerSchool that were also in Canvas, then syncing grades.