That’s the part I’ve not seen a manufacturer’s spec actually say. I’m not saying it’s not the case, I’m saying I haven’t seen it.
Found and read a Specialized one. The issue they warn about is the expansion plug being installed above the stem could make the steerer tube fail in expansion. In other words - they’re saying don’t expand something inside the tube without something clamping outside, they’re not saying don’t install a stem clamp without something inside.
Ok, playing on words a bit, I admit. It does come to the same thing - the expander plug should be level with the stem. We may disagree as to why, but not as to what.