A friend of mine has a Yeti ASR with an XTR M970 bottom bracket and chainset.

When at the bottom of the cassette and in the big ring and putting load on the pedals, the chain slips over the outside of the outer chainring, dropping down the crank arm.

It has a 73mm BB shell and one spacer on the driveside, which looks right according to the Park Tools website.

The chainring doesn’t look worn and the chain isn’t rubbing on the inside of the front mech.

Any ideas why this may be happening?