On your Lemmy account settings when logged into your home instance from the web, do you have "Show Read Posts" unchecked? That will insure any posts you've marked are read from any client won't get sent to you again when you refresh your feed later. This is a double edge sword as it can make finding read posts harder when uncheck. Some clients may do some filtering on the client side, but also enabling this server side make hiding read posts a lot more consistent between browser and other clients from the same account.

I've been getting this, but if I press Hide all Read posts then it'll go away again.

I'm having the same problrm. This was the main way I scrolled reddit. Open up r/all or my subscriptions and hide a post if read or not interested.

Just worked on your post.