@mentalstring thanks for bearing with us over all this time, sorry it’s taken so long to get to the root of this. I found the cause and a way to reproduce it today based on the data you have given and a report in Data Explorer on Meta. To reproduce:
Bookmark a topic and don’t bookmark any posts within
Delete or recover any post in the topic
It is caused by this job (added by me years ago) which doesn’t take into account Topic-level bookmarks:
So, a fix is on the way along with a data migration to correct existing records. I will post here again when I have it merged.