Fallesafe,
This is an interesting issue and I'll add this for what it's worth: I have a WM microfiber Lynx (rated to -10F) as my deep winter bag and I have the exact same issue no matter what shelter I use (typically double wall). The top of that bag is almost always wet in the morning. I can't explain it technically but I think its something to do with high-loft bags and that shell materiel. I always bring along a little towel to wipe it down in the mornings (my bag not the tent).