Steps to reproduce:

1. Go assignment grading in any course with a few students.
2. Drag a file into the form field (and wait for the upload process to complete) or select a file with the file picker.
3. Click the next user button in the top bar.

Expected result: You see the warning that you have unsaved changes.

Actual result: You navigate, silently losing the file you just uploaded.

Looking at the code, it seems very dangerous that mod_assign has its own mod/assign/amd/src/grading_form_change_checker.js, which does not use the standard core_formchangechecker, which does fully integrate with Atto, file manager, etc. (Though from MDL-54852 it seems that mod_assign does also use core_formchange_checker.)