I have noticed this too. I was very excited to have the “mark read on scroll” option added with the update yesterday.
However, as I am scrolling and marking posts read I have noticed that a lot of the posts that were marked read keep reappearing as unread. Not other instances, but the same posts.
Hard to capture this in the wild, but I will try to get images of this happening.
Also, is there a way that we can see our own posts that have been marked as read? When I go to my profile, I see my total post # under my summary, but can’t see any when I select “posts”. Comments show up, but not posts.
I have found the only way to open my past posts is to save them.
I suspect that is a Lemmy issue, but would love to see it addressed in the app if possible.