Messages remain marked as unread after they they are read (matrix).
This happens with the matrix plugin compiled from source.
Messages that were already read in both chatty as in other clients (fractal, Revolt, Riot Web), remain marked as unread in chatty. But this only happens with some rooms, so this is not a consistent behavior.
This happens with 1:1 chats and public chat rooms
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information