I have a ticket in about this but have not had resolution so far.
I am having this same issue in Infinite Campus. I believe the problem lies with Canvas, not our SIS platform of choice. This behavior is very time consuming since we have to manually categorize assignments in IC when they sync, but they only sync as students do them. Until recently, Canvas would let you sync the entire course from the start even when no assignments had been attempted by students.
My guess is Canvas is trying to reduce the load on their servers by only syncing grades that have been graded, but the result is very time consuming for teachers at my location, and one more thing we have to add to our daily list of stuff that should be automatic, like it always was in the past.