Just discovered a strange issue where achievements are being marked as completed in the future.
See screenshot: https://imgur.com/a/2yXKuJJ
Note that I did not adjust the my local time on any devices (which if I had, might explain this).
Also, in my case, these happened with achievements that had already been marked with the correct date when they actually were achieved. For some reason, at a later point they got “rewritten” with completion dates that were (and even still are) in the future.