When I hide posts in my feed I was expecting to not see them again (as it worked in RIF), but I do see them again if I refresh my feed.
I was a RIF user in Reddit, never used Sync before, so maybe this is working as intended. I hope it is a bug as for me hiding posts should prevent the post to appear again in my feed.
You mean hiding the post or making them as read?
I know the latter is completely client side, and it used to have like a workaround in Reddit to move it to their server side hidden section.