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.
In sync for reddit you could look at your hidden posts. I prefer to have hidden messages stay hidden. But hey to each their own :)
I believe that was a reddit function at it’s core