Today I noticed that refreshing my home feed (topday) was dramatically changing the results I would see, especially on posts that I knew were less than 24hrs old. I started opening the comments on the top post and refreshing home, and the post would go away. I was aware of the "Hide Read Posts" setting, so I went and confirmed it was off. I switched it on and off again to see if setting it to off again would do it, but no. Closed app, force stop app, no change.
UPDATE: There is a setting in Lemmy for "Show read posts" which had become unchecked on my account. I don't use lemmy directly -- I only use lemmy through the connect app. Perhaps lemmy reset this setting, but it's possible that the app updated this setting incorrectly? I'll remove the post if this is ruled out.