Motions don't stay active when doors and windows are on too.

The system is working as designed. If you arm both doors/windows and motions, but you DON'T exit the premises during the exit delay, the system will "downgrade" its arming to doors/windows only. The assumption is that you haven't actually left the premises and are at risk of creating fale alarms. This behavior can be disabled in System Configuration (though it may require the dealer access code to do it).

Try opening and closing a sensored external door (even if you don't go outside) during the delay after arming doors+motion and see if the motions don't stay active.