If your rig has ever been to southern Mexico particularly near the coast and been refilled there by a mobile Propane/ Gaz truck it has been filled with a mixture of Butane and Propane!
There will be a small amount of oil present especially if the tank wasn’t used a great deal on the trip back north! This oil condenses or thickens with cold temperature!
The best cure that I have found is to wait for warmer weather or fill the Tank with fresh locally brewed Propane at home as soon as I return .
I don’t refill in Mexico on the trip home or just prior to that trip !