I thought they were actually, like, certified? How can this not have been tested and fixed... shutting down the magnetron can not take long, right? Making it react fast enough doesnt feel like an intractable problem at all!
Something that becomes more apparent the more you listen (but also if you actually pay attention to diagrams of how a microwave works): the magnetron is a spinning thing with its inertia. Even if you immediately cut power to it, it still spins on its own for some amount of time. Given how much energy and wall clock time it takes to spin up to full speed, it shouldn't be surprised it needs similar wall clock time, if not energy to full stop.
But also, yeah the door pull sensor is a classic analog latch detector that has a slower sensing time than a button would by its very nature (and trying to avoid false positives from a loose/vibrating door). It's an easy thing to cut corners on and some sensors are worse than others.
(And also, safety certifications include a margin of error that it still "generally regarded as safe", what's a few extra microwaves escaping into your body among friends as long as it isn't full power?)
(And, of course, you don't absorb all of what leaks).