I have now been out on three nights with my JRB UL BMBH with TQ & UQ. On the first trip I woke in the wee hours of the morning with CBS. I figured there must be a gap somewhere on the UQ, but couldn't figure out where.
The second night was very warm so no issues.
This past Sunday/Monday I did a quick trip on the Tully Trail, and Sunday night I again woke in the wee hours to a cold butt. I got up and made sure the UQ was snug to the bottom of the BMBH, crawled back into bed and again felt cold air under my lower torso and butt.
Then I discovered the issue: I very deliberately tucked my TQ down along my sides, and voilą, the cold spot went away! Apparently cold air was leaking down the sides and under me, and not coming in underneath at all. It seems just casually having the TQ draped over me isn't quite enough. So good to have solved this little mystery!