"We provide an SDK which can be used to simulate proper push back behavior. It's up to the 3rd party developer to use it to properly interact."
Well, we might just say the same about GSX.
We do provide documentation for 3rd party developers to prevent issues with GSX pushback, and it's up to them to use it and properly interact with it, to prevent problems that only customers of their airplane are experiencing, that doesn't happen with any other airplane, either because it doesn't have this (whatever it is) system that is causing this, or because the airplane developer did the right thing, and used the GSX documentation, which clearly explains how to prevent it.
Instead, the PMDG SDK mostly covers ways to trigger all switches in the cockpit, and ways to read the CDU content. I couldn't find anything in it that seems to be related to pushback, and not even an explanation of what might causing this.
An helpful reply from PMDG would have been something like this:
"Yes, it happens because the airplane simulates the auto-steering-computer
(I'm making this up!!), which everybody knows must be turned off before pushback so, it's enough they turn it off when pushback starts, and it will be fixed".
If they said so, we might have used some command in their SDK to turn off that thing, something that you could probably do it yourself as well, since all custom events in their SDK are related to some switch you can operate in the cockpit.
