What the subject says; After a certain (not consistent) amount of time, the MacOS desktop client stops auto-scrolling when there are new messages in a window. The new messages are available if you scroll down, but you have to manually do so.
There does not appear to be specific UI behavior that triggers this. For example, it occurs (eventually) even if you never manually scroll the window back up at any time.
This is Version 5.1.0 (19937), on MacOS 10.15.7 (19H1417).