I think this may derive from some very odd ATPL questions which don't set the scenario properly, I'd be interested to hear what the OP is driving at. I wonder if the reference to constant mixture setting means 'don't touch the knob', or does it mean 'leaning the engine appropriately as air density decreases to maintain a constant air:fuel mass ratio'? Probably the former, I would guess. CSE's old piston engine performance notes are likely to blame here, several challengable statements that were never tested under the old syllabus but are now being plucked verbatim by the examiners, and being looked at closely for the first time.