If you insist that deploying causes HIP loss
I'm not insisting on it, the rules are insisting on it. Unless you have a rules quote that explicitly states that deployment does not cause loss of HIP? ... No? I didn't think so. Do you have a rule (that isn't about cave complexes) that says HIP is not allocated to a unit, but just kind of randomly exists in a Location and any passing unit can just take it whenever it wants? ... No? I didn't think so. So what does COWTRA tell us? It tells us that units may be granted HIP status, and therefore if that unit no longer exists (as a deployed squad no longer exists) then there is no more HIP status to worry about. Why is this even being debated?
Also if the SSR allowing HIP allows the MMC (and any SMC/SW stacked with it), when the MMC leaves without that SMC do you believe the SMC loses HIP (becomes unconcealed) as well? It was granted HIP as part of a stack that no longer exists.
Now you're just being silly. Stacks are not units
[EXC: dummy stacks]. Stacks are not granted HIP status, the units (and portaged SW etc.) in the stack are granted HIP. Being stacked with the MMC was a pre-requisite to being granted that status but does not continue as an ongoing requirement (as you pointed out in your post above). The individual units (and any unportaged SW, if they're dropped) may (or must, in the case of a dropped weapon) forfeit their HIP at any time without impacting on the HIP status of other units present.
Less clear is what happens to a SW that is allowed to be HIP (because it's being portaged by a unit granted HIP status) that is
transferred to another HIP unit in that Location. My reading of E1.2 suggests that it will remain HIP, but I'd like something more definitive. Also, if you had two HIP Emplaced Guns in a Location, and the crews decide to swap guns, does everything stay HIP? This sort of thing is out there on the bleeding edge of situations that the rules do and do not cover ....
he didn’t say anything about the leader.
I would suggest (as I've already suggested) that he didn't say anything because he didn't need to; the leader has done nothing to forfeit his HIP status. (This is all assuming that there is no enemy LOS to the HIP stack, of course.) The
squad forfeits its HIP status by virtue of no longer existing, and HIP status is not something that one unit can just voluntarily pass on to another.