We are having the same issue only the instructor started with Point turned on and Simple Direct Grading. The grades in his second and third forum aren't saving at all and in his first forum, four out of five grades saved, but the fifth keeps disappearing. He marked his fourth forum over the weekend and the grades saved. The settings in all four forums are identical and he is putting in the grades via the "Grade Users" option.
He is able to manually add a grade directly in the Grade Book and have it stick, but it doesn't show when he goes into "Grade Users" in the Forum.
We switched the Point to Scale in one of the forums impacted, saved the change, then changed the Scale back to Point; I (Admin Role) was able to add marks for all students in that forum and have them save. The instructor repeated the process in the other forum and was able to enter marks for one student in one forum and have it display to the student, but not for the other students in the forum; for the other students he ended up adding the marks directly to the grade book. The instructor tried going back into the forum I entered marks into when testing to adjust those marks and was not able to do so; he ended up adding the correct marks for that forum directly to the Grade Book as well.
Switching from Scale to Point seems to work, but very inconsistently.
We upgraded from ver. 3.7 to Moodle 3.9.7+ (Build: 20210520) in July.