When grouping by assignee, the view is limited to 50 users. Not 50 users at a time, but 50 users total. The truncation seems to be arbitrary as well, rather than picking, say, the first 50 users.
I was informed that this is for performance reasons, as large queries can be quite slow if not indexed properly.
If there is a way to split up the queries and paginate the resulting view, hopefully that can mitigate the aforementioned performance issues plaguing this otherwise useful view!