So why on the previous GSX build I didn't have the issue under the same configuration (FSX-SE and PMDG 777) ?
Nothing has changed in the GSX pushback procedure since several years and, of course, I'm sure you don't see this in any other airplane.
What might have been changed instead, is the airplane so, whatever is causing this, might must be something that has been added or changed recently.
As I've said, every airplane developer has every means to verify if the airplane is being pushed by GSX, so they can easily turn off whatever non-standard simulation they are running during that time.