Just found this post. Copying and pasting a comment I made elsewhere:
I just started having this problem in the past few days after it working for a long time.
I've cleared all my data and reset all my settings. On the web I have it set to not show read posts. In the app I haven't set to mark read on scroll. And to show read posts as dimmed.
But no matter how much I scroll nothing gets dimmed, and when I refresh everything is still there.