That is pretty much the scenario I envision. I've seen it on TR6's which have the same ARV plumbing and same basic ZS carbs.
That sounds like it might be correct plumbing for the ARV.
If you follow BH19, BH21, BH16, BH3 and BH4 you'll see the basic ARV plumbing.
https://trf.zeni.net/TR6bluebook/49.php
When you turn the ignition key to OFF, killing the motor, the ARV gets power from the ignition switch. The ground on the ARV comes from the oil pressure sensor and during the few seconds that it takes for the OP to drop, the sensor provides that ground, completing the circuit that opens the ARV to the manifold depression. During that interval the plumbing is open from one of the carbs' float chambers vents, to the carbon canister, to the intake manifold.
Just to be clear, each of your carbs have 2 float chamber vents.
This one identified in red is the one for the ARV plumbing when the engine is idling. The other vent for higher speeds is the round brass lined hole on the carbs' air box mounting flange.