While I don't fly helicopters nor do mustering I have been told that during mustering mostly it is not high power used. Lots of manoeuvring yes, but cows don't run very fast.
Detonation I would expect would cause damage to the pistons (holes), what seems to be reported is accelerated wear of the cylinders. I assume this wear is the sidewalls and/or piston rings possibly valves, guides and seats could be included.
So I think it is something other than detonation.
Is the issue similar across carburettor and fuel injected engines?
AWB 85-024 Issue 1 - points to exhaust and guide as the area of damage and wear.