yeah right, cos I have never built a bike up before.
He didn’t say that. You have a product which is successfully fitted to literally millions of bikes, 99.9999% of which have no problems. Yours fell off after 3 months of ownership. There’s no inherent issue with the product, neither generally nor in this specific case. Ergo the obvious cause is that it was fitted wrong.
I had one come loose, it didn’t actually fall off, it rotated slightly on the splines, which is probably more damaging. Just one of those things, it obviously wasn’t tight. Be glad it’s done no damage, shrug your shoulders and move on.