Brain saw android, assumed app. My fault. Still, the problem has been persistent for me across all fediverse instances, not even just lemmy. On android and PC (Linux and Windows.).
No worries... My point though was that maybe you could try a different browser because, as evidenced by my screenshot and anecdotal experience, it works in at least one of them.
I'm guessing you are getting what I am meaning. Like if I use the inline content viewer thing on either images or video or whatever content, the thread is NOT marked visited. The only time it's marked visited is if I either click the title link or the comments button.
Just tested it again in Brave specifically with clicking the images in the inline content viewer without clicking the title or comments button on old.lemmy.world
Here's what it looks like before I use the inline image viewer.
While using the inline image viewer.
After inline image is closed
So I've viewed the content, but it doesn't count as viewed because I didn't enter the actual thread itself. The only way I can get it to show as viewed is to click the title link or the comments link.