I synced my deck for the first time in a couple of days and 500+ more review cards (incorrectly) showed up as being overdue. I went to Browse to see what the issue was and Anki has noted my completion of the cards, but still says they are overdue (e.g. images attached: say Due 12/31/2020, but Latest Review was 01/07/20). All of the reviews that they are not registering at the proper intervals seem to be confined to the days in between my syncs (last two syncs were 12/30/20 and 01/08/21 – all the implicated cards fall in this window). In the future, I will be syncing more often. I have updated and restarted Anki as well as restarted my computer and synced from my phone. Any help is greatly appreciated to get Anki to re-acknowledge these reviews!
If a card is changed on two devices at once, the most recent change wins. Perhaps you performed some action like moving the card to a different deck on one device, after reviewing on the other. To prevent conflicts, please always sync at the start and end of a session.
In terms of recovering your reviews, you may be able to restore from an automatic backup on the device that made the reviews. Restoring an Automatic Backup (Recovering from Data Loss) - Frequently Asked Questions