It’s not common for any alarm system I have ever used. All these have allowed me to start the arming countdown WHILE exiting via the defined entry/exit. Of course if I tried to arm the system with another entry/exit/window open they would throw a wobbler. They all managed fine to stop me from leaving non defined exits open. If I managed to leave the defined entry/exit open (or took too long getting out) the alarm would sound. If I walked away having done this I’d be pretty stupid.
I don’t want to use the app after I have exited. I want to get into my car and drive away, not faff with my phone waiting for the ring app to load. I have a perfectly functional keypad just by my front door so I’d rather use that. I just need ring to fix the dumb behaviour of the system or at least offer the option to have the exit open while starting the countdown.