It looks like either: a fuse running just above its rating, where it gets very hot but fails to blow. Either the fuse is underrated or the machine has developed a fault. Has the fuse actually blown?
It could also be a loose wire causing arcing inside the plug.
Interesting. I assumed that a fault generating enough heat to melt the plug would trip either the RCD due to current going to earth or the MCB on over current.
It looks like either: a fuse running just above its rating, where it gets very hot but fails to blow. Either the fuse is underrated or the machine has developed a fault. Has the fuse actually blown?
It could also be a loose wire causing arcing inside the plug.
I don't think an RCD would trip for either case.