Currently if instructors create a group assignment, and a group set, they can enable self sign-up within that group set. This is desirable as often they want students to self-organise into groups of common interest.
It is clear that students changing groups after submissions have been made can cause trouble and confusion in Canvas.
To prevent this, instructors have to remember to manually edit a group set and disable self self-up on a date that comes before students start submitting.
When enabling self sign-up, allow the instructor to set a date and time at which self sign-up becomes disabled or closed. For example: self sign-up might become disabled on October 10, at 11:59pm.
If this date is set, place an event in students' to-do lists and calendars, set for that date and time, reminding them that they have to join a group if they haven't already. Once they join a group within that group set, remove the to-do list item.