I have noticed this for a while, but old post that i know i have liked, are not liked by me or almost anyone. Is there some like limit tied to the account? or are the likes vanishing for some other reason, it seems to me that the likes that stick around are from those i see far less often and the ones that seem to be missing are from the frequent fliers
I looked into it and this is what I can tell you. Firstly, we have around 2.25 million forum likes in the db. Forever ago, I created a per user limit of 1000 likes per day. The way it was reset was a daily cron resetting the counters. If you were to go past the counter the site would act like it worked and render the like on the post (if you refresh it would be gone because it wasn't recorded).
Here is my theory, the daily cron has other responsibilities. If some other operation failed before the like reset, the like reset would never happen. This would support your observation of frequent forum users being affected.
I checked and it's resetting fine now, I'm going to make it self healing with a ttl on a per user redis key and update the feedback on the UI when the cap is hit.