Ive had this before if its the same thing, its a screen switching issue that @emre had to fix. The user is logged out but something is stopping the pos screen from changing back to the login screen, the result is it appears to stick on the pos screen with the main menu and keyboard buttons disappearing when in fact the user has been looged out
I think it had something to do with other rules conflicting, in on scenario to over come this i added the logout action woth a delay of 1 second, this allowed all other rules to fire and complete before then logging on. All you notice on screen is that 1 second delay before logging out and the login screen appearing. I tried it with a 0.5 second delay but this wasnt long enough for the other processes to complete, the lowest delay i could get to work was 1 second