I have seen a few photos of worse and is not a rare occurrence.
Have you perchance been plugging in with the power still live? If so, the converter has an inrush current (capacitor charging current) that causes pitting and attracts dirt and then you end up with a high resistance connection that can overheat and melt. Sometimes the retention force inside a cord connector, pigtail adapter, pedestal or home receptacle can be weak which can contribute to the situation of dirty plug blades. It's good practice to keep an eye on the shore power cord's plug blades (and adapters) and clean them with emery cloth or sandpaper as needed. When you plug in live, you can hear a zap and in the dark can see a flash.
The problem with cord connectors and receptacles is that you can't see inside them to tell if they are pitted and dirty. Have come across some pedestals that have loose and crispy-looking 30 amp receptacles that just shouldn't be used, but sometimes you may not have a choice. I use a 30 to 30 amp pigtail adapter in those cases so if it overheats, I'd only lose the adapter. Reducing loads to a min. will help too. Be wary of a receptacle that is obviously loose. I had to use a stick once at a pedestal to hold the plug in place (no other sites with better pedestals).
In the beginning, I had the connectors on two cheapo variety 15 amp extension cords go up in smoke because I was plugging our TT in with the power live. Once I started turning the power off, it never happened again. I ended up installing a dedicated 30 amp recept. for our TT in the carport along with a disconnect switch next to it.