Just noticed this today as I finally completed the Zereth Mortis questlines I started back in 2022. The completed quests for Secrets of the First Ones achievement only showed completed for what I just did today. The other half that were done back in 2022 were greyed out. Was unable to get the achievement. Same for Tales of the Exile. I found the last 2 books today and no achievement. The other 5 books have been sitting in my inventory so clearly they were found. But no credit. Frustrating.
I imagine I could find many more like this. I have not looked beyond Shadowlands achieves so cannot speak to any other expansions being affected. I really hope they figure out how to fix this. It is too much to re do with alts.