Easy enough to reproduce - simply change the date on a movie to match 1969-12-31 and it will stop showing a release date, and instead just show a year, then put it at the very end of the list when sorting By Release Date.
The data still exists on the file, and locking it makes no difference (so changing it by one day has it appear roughly in the right place, but if you don’t lock the date the next time it refreshes it will move it again).
Posted in server as it happens on all clients using the API as intended, so a likely case of transforming into a unix timestamp and treating 0 as meaning on date (while it can also be valid etc) 
