Doesn't matter, nothing changes. If you have a strange effect something is confining. If you don't know how it is confining you have to entertain all possibilities so you need the stats at each phase
These trees aren't very dense or hot and they can't touch for the same reason. Every leaf that falls from the edge has to be replaced at the center. https://en.wikipedia.org/wiki/Crown_shyness
Near the Quanta things get discrete so coagulation gets chunky, but nothing changes in principle even down at 10^-3 Torr. In fact, the high-pressure, higher-heat phases of Hydrogen aren't that interesting, but the high-heat "moderate" pressure ones are. There is no escape from consequences of the material itself.
> The propellant is extremely rarified and not particularly hot
This type of statement is where you hide blind spots from yourself. You are going to extreme effort to confine, detonate, an exhaust fuel at just the right time. I would wake up everyday asking, how is my fuel less like a rarified gas and more like a hot, dense fluid. Like, your answer is in some tricky phase of the noble gases. There will be some kind of confinement or effective erasure of expected confinement. It will look like all secondary effects once you discover it, like, "oh, neat phase, water also expands when it freezes, now it makes sense". Why is this whole topic mystified? It seems only as hard to understand as air conditioning.