To be honest I’m a little confused as to why they used a 6amp rated diode when, if the blue wire on your bike is the same as mine and on a 7.5amp circuit. Or if like you thought, is the wire for the headlight and therefore on a 15amp circuit making it an even worse choice.
It is still worth finding out which fuse that wire actually goes to just for future knowledge but I think I would be looking at a higher rated diode such as this:
https://rover.ebay.com/rover/0/0/0?m...2F192608781257
These are basically the same kind of ‘blocking’ diode used on those little battery charging solar panels to prevent the battery sending voltage to the panel instead of the other way round.