Grades disappearing or overwritten by students

Hello Moodlers.

I have a weird bug  appearing in Moodle version 3.11.2+ (Build: 20210805) regarding grades.

We have more than 6 cases where lecturers graded the students assignments, provided a feedback and few days/hours later the grade disappeared but the feedback comment is still there.

Checking the Grade History I found these actions. Please note the grader

The Teacher graded the assignment on 23-11-2021 at 7:34PM with grade 0.80

Then I see another entry, the grader is the Student and it seems that the grade was removed afterwards

I have searched over the internet and the forums, the only similar case I found is a case for Moodle 1.9 but no further information.

Could you please help us, or guide us as to whats the issue here?