I am in Northern Wisconsin with somewhat similar weather and have not seen any problems like that in any of my rifles. It seems strange that the rifle is doing this. Just a few thoughts, and I am not saying this is a cure.

When you're done swabbing the barrel with solvent, do you clean out the solvent as well? Normally after the solvent patches, I like to take a isopropyl alcohol patch and patch the barrel, then some dry patches to insure the barrel is free of all oil and solvents. After that, I run my oil patch down the barrel.

Next question would be what brand of oil are you using to protect the rifle? IMO some gun oils are good, some are not so good. I use Birchwood Casey Sheath a.k.a. Barricade. Another good one is Rem Oil with Teflon. Also Breakfree CLP is good, but I use that more of as a cleaner then a final protector...

I am sure you're very knowledgeable in the care and storage of your rifles. So this is why I wonder what could be causing this. If it continues, I would discuss your cleaning schedule with Thompson Center Arms and see what they suggest.