We live in the country, use propane, and have used whole home humidifiers for nearly 30 years. We use 3, one for each floor. Ours are larger humidifiers than I think you are referring to. They have never been the cause of a problem like that. We even have a heater in the garage with a flame, and with all the humidity in a garage (rain, snow, etc.), it burns perfectly. I really don't think humidity in a house would be the culprit. Humidity is really necessary for the house and the people inside too.
We did have a problem with the pressure regulator outside, the water heater was not burning properly (orange flame which caused sooting). The propane company offered to come out test the pressure regulator on the outside of the house. It needed adjustment (it was low) and it fixed the problem. They did it free of charge. They also checked the all appliances inside the house and adjusted them to the proper pressure. They had all been set originally, but still needed readjustment. They recently came out and did the same thing at our sugar house (where we produce maple syrup). It needed it (even after an initial adjustment). When we are boiling sap in the sugar house, it is very humid, yet our gas stove works perfectly in there (when the pressures are proper). Maybe that is where your should do some checking first. Hope this helps.