We have a 2018, NL 8-11 and we haven’t had the issue that you are talking about. However, I don’t know if you have the “ breathing material” that came in our 2018 model.
We did the tour of Northern Lite in Kelowna and they showed us ( wasn’t pointed out when be ordered our rig at the dealer) that they have this breathing material that my grandson calls “ichiban soup “ etween the mattress and the floor of the “ bunk area” .
We haven’t had an issue yet, however we haven’t camped in the cold yet either. If this material isn’t under your mattress, send NL a note and ask about it. I’m pretty sure it is new to there rigs as they “ insured” that they showed it to us and explained what it was for .
Soup.