I reckon you’re right there Darren- I see this kind of thing quite frequently in my work where cable connections are not done up properly and heat generated by the current is enough to melt the insulation.
It has been mentioned many times on here where the crimp terminals in the block connectors have either a build up of corrosion or a loose connection leading to high resistance generating heat so the wire and or the connector block itself melts- this is particularly prone to happen on the alternator and regulator/rectifier wires.
Going on from this the under seat fuse holders are basically a series of uninsulated 1/4” faston crimps held in place by the box they sit in. The underside of this fuse box is open to the elements and sits in just the right place to get a bit of spray and road crap from the back wheel leading to, if not cleaned up regularly (yep you guessed it) a higher than wanted resistance.
So as many here have said before me, and certainly as our bikes get older it is definitely worth making the cleaning and applying of some silicone grease to the electrical connections part of your yearly maintenance, especially if you often ride in poor weather.
In addition, the fuse blades themselves will also dull up and corrode over time so either give them a good clean or for the few pence they cost replace them for some new tight fitting ones.
You can even be a bit flash and get some ‘blow glows’- it’s almost worth having one fail just to see it work
https://rover.ebay.com/rover/0/0/0?m...2F291052958453
Personally, as well as the spare fuses that are housed in the fuse box, I also carry (in the under seat tool box) a spare 30 Amp Maxi Fuse (the under the tank one), a new General (Yellow) relay and a puncture repair kit because any one of those can be a show stopper leading to a long wait for a recovery truck.