Anyone have any ideas what and how caused this damage to the stay in a single 24km ride?

I assume it must involve the chain .

Swinging arm hitting front mech?

Looks a lot like the front mech gouging in to me.

Chainsuck?

Robot mice in your shed. They come out at night and feed on aluminium.

During the day they hide away and dream of electric cats.

Titus El-Guapo? front mech hits the chainstay yoke on full compressions? seen it a few times. If you just set the mech up to work well it will hit, you need to nudge the lower limit screw in a tiny bit more and make sure cage is perfectly straight. Might result in a tiny bit of rubbing but should work ok.

shifter – Member
Swinging arm hitting front mech?

This appears to be it. The mech is not very low considering that ring in the pic is my outer.

Is there similar marking on the other side of the front mech.

If its an El Guapo,my mates did exactly the same.

It is an El Guapo!