iOS: Students cannot see comments on marked work

Question

I'm working with a teacher to resolve an issue in Classroom & Docs for iOS.  He has assigned some written work to his students, and they have completed the assignment and handed in a mix of Google Doc and PDF files.  He has then commented on those files and returned the (unmarked) assignment.
For PDFs, girls can open their PDF and see a highlight where he left a comment, but not the comment.  If they then press the 'open in new window' button at the top right, the file opens in Drive and they then get a speech bubble button at the top right that they can press to see the comments.
For Docs, girls cannot see highlights or comments upon initially opening their file.  If they then press the 'open in a new window' button, the file opens in the Docs app.  The student has rights only to view the file, and cannot see the comments or any indication that there is a comment -- the view comments button at top right is greyed out.
I've seen a couple of threads like this back in April, but nothing since.  How do I resolve this problem?  It pretty thoroughly defeats the purpose of comment-marking Docs work in our 1:1 iPad environment.  Has anyone got any ideas?
Thanks,
Jonathan