I have an XTX and it happened when I used tape to hold together the cush/hub during assembly - when I peeled off the tape, the silver paint went with it. Word of warning: the same silver paint is used on parts of the engine - at least on the left side - but instead of raw aluminium, it's the (slightly darker) paint used on the rest of the engine. I was able to blend it in, so it looked alright.
I use a barrier coat (silicone oil) to make the engine easier to clean. I tried WD40's silicone oil but quickly learned that it has citrus oil too - which stripped the lighter silver topcoat.