I wonder which started first, the contact erosion could have started after everything got hot from a loose cable connection and the switch contact moved position when the plastic melted/burned.
I have seen other problems with the metal Lewdon 100A switch fuses, went to look at a fault at a pub/restaurant that only had a single phase supply, a large distribution board was connected via the switch fuse. This was new and had been fitted 6 days earlier by another company as the old switch fuse burnt out. I was called to 'check the electrics' as the cutout fuse had blown. Looking in the new switch fuse the connections were all loose and one of the new 25mm tails was clearly heat damaged. I don't believe the terminals were left loose by the installer but had worked loose from the heavy thermal cycling. The brass terminals the installer fitted the tails to in the 4 100A blocks, including the 2 new ones he fitted to cut out the heat damage from the outgoing tails were all tight.
With the power on I measured 141A being drawn and the wiring and fuse soon started getting hot.