I just tested it and it seems to work consistently for me. The way it's designed to work is any post you scroll past (eg. the post has to be scrolled completely off screen) will be marked as read. If you think it should work differently or if this is not what you see happening please let me know.
Also if you could share a video that would also be helpful for me to look into the issue since I can't reproduce it on my end.
Update
I reviewed the code for this and I found a minor flaw however it should still be marking all posts above a certain point as read, it's just the "line" it uses to figure out what to mark can be unpredictable. Eg. an item can be scrolled a off screen and still not marked as read until a certain amount of things have been scrolled on screen. I've updated the logic so it items scrolled off screen are instantly marked as read. This change will go out in the next update.