I've got "mark read on scroll" enabled as well as "hide read posts".
Same. Those features have never worked for me either. Equally effective as pushing the crosswalk button at a traffic intersection, or pushing the "close door" button inside an elevator. Useless. Best I can do on Lemmy is click the "refresh button" sometimes or come back a few days later to get a fresh feed.
I'm convinced that people with excessive levels of wealth have fewer brain synapses than the rest of us living real lives of constant work & negotiating & constantly coming up with creative survival strategies.