It sounds like the ring locater pin came out of the piston and cut a line in the jug.
How did you discover it?
Are you assuming the pin caused the groove or have you had the piston out already to see the pin coming loose?
The pin is pressed into a tiny hole drilled in the piston ring groove. If the pin doesn't fit tightly, it can work loose and carve-up the liner.
This is common in CNC machining when tooling is used beyond it's life or just a defective drill bit was used on a run, but definitely not by your break-in or fuel.

Being the engine is still under warranty, tell them you discovered a groove and think the pin caused it, but don't disassemble it any further.
Chances are they've seen it before and a new piston and liner should make it good as new.

Kirk