I spent some time trying to replicate it, but I wasn’t able to do so.
I checked Chat::MessagesQuery, Chat::ListChannelMessages and the controller action through our specs and manually on a local instance too. The page_size parameter is taken into account as expected, as well as the direction parameter.
@Nima1 are you sure to use the proper endpoint? It should have api in it, like so: /chat/api/channels/3/messages?target_message_id=40&direction=past&page_size=5.
Another thing I haven’t checked is if it behaves differently on stable. Is your instance up-to-date with the tests-passed branch?