Phil, this seems to be a problem with a couple of Yamahas when the suspension is either too hard or too soft (wears either top or bottom of the slipper), combined with a kinked or too-tight chain and/or a smaller front sprocket. It doesn't take much...

References: I chopped out the top of a 426 slipper in 400km . Also, WR250Rs are noted for chewing the bottom out of the front slipper if the chain is too tight (even slightly), or a smaller front sprocket is used.

Imo, run your chain a bit loose if you can (watch out for derailment), with stock front sprocket and correct spring rate for load.

Sorry if I'm preaching to the converted. Just my experiences, that's all.

Hope you get it sorted mate.