The purpose of the <Reopen Assignment> function is to provide a new opportunity to complete the activity, whether for one, some, or all students in the class.
When the assignment is reopened, previous grades are automatically deleted, as indicated in the on-screen confirmation message. This happens because the system assumes that students will complete the activity again from the beginning.
Recommendation:
To avoid losing previous grades, it is recommended to manually select only the students who will retake the activity, instead of reopening it for the entire class.