IS enabled or disabled made no difference.
It could be the IS units themselves. perhaps more susceptible to the shutter vibrations. the masses as well aren't all the same and it is focal length dependant. that results show that. even with the same lens, the results vary depending on focal length.
This seems to be the case with some of Canon's cheaper STM units (perhaps eureka moment here)
https://www.the-digital-picture.com/News/News-Post.aspx?News=11407
They discovered or mentioned that with the EF-S 10-18mm STM lens the IS unit has a different "lock":
"When the IS is Off, image stabilization optics are locked in place with a spring suspension mechanism rather than a center lock mechanism. "
this would make it susceptible to the small vibrations of the shutter. Then perhaps with the IS unit being ON it would make little difference because it can't react that fast to the small vibrations of the shutter (theory).
This would also explain why the EF-M 32mm did not exhibit as much of a problem.
the tests were repeated and performed three different ways, different orientations of the camera to tripod head, with and without 10 second timer.
if it was a focusing precision issue or even a tripod issue than the EF-M 32mm would have shown dramatically different results, as well as the EF-M 55-200mm at 55m.
Also, the test was done in such a way to also try to mimic handholding to a certain degree by putting the camera in portrait mode. but in reality, it didn't make much difference.
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Takeway - I guess you could argue that it's not "shutter shock" but it's obvious that it's IQ loss derived by the fact that it does have shutter shock paired with the EF-M lenses that most likely have spring based lock mechanisms. In reality it really doesn't matter - the shutter is causing the problem.
I seem to recall this being a similar problem with other IS's on cameras that did have shutter shock years back, but that information is pretty scant to find these days.
I'm still pretty confident to call it like it is, even though I think semantically you can argue the term. The M6 Mark II is shipped with these zooms, and exhibits the problems when using the mechanical shutter and these zooms.