It is, just as described before, that around 140ish km/h, as long as you are using active cruise control(the lane centering is not available above 130, so then MG Pilot basically becomes ACC), the gear box will not use top gear, at least is very reluctant to do so. Which means that instead of a little over 3.000 rpm, which the engine revs without using cruise control, it will rev somewhere below 5.000, with all the noise and consumption associated with that. Generally with MG Pilot the car seems to hang on more to lower gears, but at this speed it is really obvious. Especially given that without MG Pilot, the "normal torque converter gearbox" in the ZS is way less prone to droning rev ups at slightest press of the pedal compared to eCVTs used in say Toyotas.
Given 90% of my driving is city highways or urban commute, and I've had the car little over a month, I don't have much experience with MG pilot on our Autobahn so far, but 140ish is what traffic tends to travel in Austria, certainly at some point the cruise will have to go into top gear as speeds above probably 150/60 would run out of revs for sure, but that would quickly become expensive here (also, the ZS seems to have a very precise speedo, like very little difference to GPSS speed, bare in mind when passing speed cameras). And below 120, the electric engine is supporting, so probably less of an issue in total, so I guess that is why the 140 Speed window seems so prominent.
It really seems like some miscommunication between cruise control and gear box at some points/rev ranges, that should easily be fixed with an update (hopefully). Given the petrol engine alone has it's maximum torque at 4.500 rpm, probably cruise always wants maximum acceleration available (which would be at max. torque), but that is certainly not necessary when cruising at a steady speed.