Our district is also experiencing this issue with Google Assignments. We aren't having issues with Google Assignments that are newly created in a Canvas course. We are only experiencing issues when an Assignment is copied, sent to, course import from another Canvas course or imported from Canvas Commons. When any of these "copy" functions occur, the Google Assignment file that is linked cannot be changed. If a teacher tries to reattach a different Google file and then saves the assignment, the file in this Google Assignment reverts back to the file that was "copied".
In addition, if a teacher uses the same exact Google File in one of these "copied" assignments, but makes edits to the file, the edits are not following the file in the Google Assignment within Canvas.
The strange thing is that we can successfully "change" the file or "edit" contents in an attached Google Assignment file successfully as long as the Assignment wasn't copied from another Canvas course.
We can reproduce these behaviors between courses (same teacher/same Google Drive) as well as between courses with different teachers (different Google Drives).
I suspect this will cause huge issues in the Fall when teachers using this new Google Assignments LTI 1.3 tool are copying entire courses for the first time.
Has anyone found a work-around to this issue?