We may be confused by what each other has experienced, but the water vapour is just doing its own thing, irrespective of our understanding.
My understanding (and flawed it may be !!) is that the insensible relative humidity on the surface of our skin in the still air within our sleeping systems maxes out at 76% RH. If we get a little over-heated as we get settled in our sleeping bags, then we our body may perspire in a natural effort to cool down, and that RH of 76% can quickly turn into an RH of 100%, but let's say we don't get over-heated for now.
That RH of 76% is much higher than the RH of the air in the insulation a little further away from our skin. Humidity will naturally move from areas of high RH to areas of low RH, so that vapor close to our skin moves out through the insulation. As the RH next to our skin drops, we perspire insensibly a little more to raise it back to 76%. So the cycle continues all night long as we inadvertently "push" moisture out into our insulating layers.
As the moisture vapour moves out through the insulation, further from the body (heat source), it runs into cooler and cooler air. Increasing the amount of humidity and decreasing the temp means that at some point the dew point is reached where that layer of air near the outer surface of our sleeping systems can not hold any more vapour, and some of the vapour condenses into liquid.
Gore-Tex only transmits vapour, so if the dew point is inside the Gore-Tex, the trapped liquid will not escape out through the Gore-Tex layer. Later that night, as your metabolism drops and your heat output drops, and the night-time air temp. drops, the air inside your insulation around that liquid water may well drop below 0C, and that liquid will become frost.
This problem gets worse as more moisture is pumped into the bag on successive nights, if it is not dried during the days. (On one 13 night January trip in a cold tent with no outer shell on the sleep system my -40C synthetic bag accumulated almost 4.5kg of crunchy water
If I may, DH, I think that Gore-Tex will still pass gaseous moisture even well below freezing temps so long as that moisture is still in a gaseous state (above the dew point), and so long as the pores in the Gore membrane are not plugged up with frost. I agree completely with your statement that a fairly thin insulating layer, as in a light wool blanket, will serve to keep the air temp immediately outside your main sleeping bag's shell slightly above 0C, allowing humidity to be pushed out of your main insulation, Some of that moisture will condense and freeze in the light wool blanket, but that can be largely shaken out next morning.
M55, could it be that the temperature of the outer layer bivy bag of your sleep system in the "trench, tent or hut" did not dip below the dew point, or that you did not push much moisture out through your sleep system between opportunities for your system to dry out?