[gtfoxy]

Being in track mode impacts shifting.

Also, the PTM modes, from wet to race impact back shifting & torque modulation.

I have never done your exact MPH/ gear test, but from what I know of planetary sets what you may be asking it to do, if not a function of prior mentioned PTM modes, may be a mapping juncture where GM said, “Yeah, at that torque request & gear change it puts too much stress on the planetaries, clutches & shafts, so none of that.”