I think there is a logic problem here. This rule won't fire until the second person arrives, not the first. When both are gone, and HSM is armed, the arrival of the first causes the Required Expression to become true -- but it was not true at the time of the event so the rule will not fire then. Upon the next arrival, Required Expression is true to begin with, so it is triggered, and HSM is disarmed then.
However, I did discover a bug related to the trigger of any presence arrives. as this is triggering when the first trigger departs. This bug will be fixed in the 2.3.2 release, now in beta. And, consistent with your findings there is a separate bug related to HSM Status in a Required Expression. This one meant that it failed to detect that the Required Expression became false when HSM was disarmed. This too will be fixed in the next release.