If you edit a file using joe and then split the screen, the top window will scroll down without any problems. Scrolling up, however, it does not, you have to refresh the screen to get it to display properly. Also, sometimes when you cut and paste, you have to refresh the screen to see the changes properly.
This has been assigned to a developer for further review.
I cannot duplicate this behaviour under gnome-terminals or xterms using Joe 2.8 in the latest tree. If it was a ncurses problem (most likely), it appears to have been fixed by newer versions of the curses libraries than shipped with 5.1.