Well, the siren going off can be easily tested by setting off an alarm (make sure the account is in test mode)
Alarm.com remote disarm commands can disarm a panel in alarm status.
I would bet the disarm command wasn’t received by the panel until moments after the window opened. Your panel will announce “Alarm, [insert zone descriptor]” before the siren. This would result in an alarm signal being sent and the panel being silenced.
There is the “entry delay” which is when you first open the door, the panel beeps and you are supposed to disarm it. You can also program a “dialer delay” (Q35) which can be enabled or disabled for each sensor. The dialer delay means the panel won’t send a signal to the central station until after the alarm has been sounding for that amount of time. The purpose of the dialer delay is to help prevent false alarms but it decreases security.