Hi Duno,
I’ve just checked follow your instruction.
In the _postmeta table, there are still some posts with rating. These posts display stars in front end and no problem with them.
Posts lost rating are those there are no rating record in the _postmeta table.
For some posts, I can rate them. And it display rating correctly in front end for a while (even when I clear all cache manually). But when I check the _postmeta table, there are no rating records for these posts.
That explains the rating for these post gone after 1-2 days.