1/ “My guess is as good as yours”:
2/ can you reproduce the error?
3/ spot your user name uploaded sequence which appears to have been taken 6 Nov.'21 ; thus likely rule out flat battery caused revert to factory settings; plus that ‘oldest’ settable year on Hero9 is 2020, on hero(2018) 2018 ; Max launched in between, therefore unlikely to (re-)set calendar to before 2019? (but please check!): wrong date may then not be caused in the camera?
Next cause would be somewhere in processing : clock in computer / tablet etc lost track of time? Software used to transfer inadvertently set different date?
Not sure what we’d learn, could you view Exif dates in downloaded files? Off the top of the head the Hero9 pics have seven exif date/time entries scattered over various sections ; one shows GPS-date+time taken : is that one correct? Is it correct if you access the SD-card with some exif-tool?
Met vriendelijke groet (Dutch for 'with friendly greeting),