There is a lot of current going through the switch.
The spades on the back that you clip onto are riveted to the switch and over time, the rivets loosen a bit. This creates more electrical resistance and resistance means heat is generated
I had the same problem. I took out the switch, cleaned the spade metal around the rivet until fresh copper colour, then cleaned the rivet and the landing the spade rivets to. Then I soldered the rivet/spade landing area.
Problem went away
If you post a photo of the rear of the ignition switch, I will highlight the areas that need to be cleaned before soldering
Dennis 69 OTS