The book wasn’t previously in my library before marking it as Finished, as an extra note.
Update: Just realized that the activity feed is updating as if I had finished the book then. Guess I’ll leave this open just for discussion: feels like an activity feed should show current updates, i.e. me marking the book read today, instead of showing the update based on the date marked, if that makes sense?
I’m not sure I fully get it by your explanation what has happened but:
If you intendedly marked a date other than today, then yes, the update will show the date you inputted instead of when you submitted the entry. This is the current behaviour although there has been some discussions whether has to be like that or not.
Actual time vs Entry time has been discussed here:
If on the other hand you haven’t written any date, then it might be a similar bug to: