It's normal. I've seen slight marks or groves on some of my engine from the bottom of the rod rubbing against the casing ....OS and other brands.
That's why you break it in slowly. That's one of the reason you won't get maximum power until after a gallon or two.
There's drag coming from somewhere.

I threw a rod only once on a nitro engine..it was on a car engine. I lean it too fast. Only a couple tanks after break in..but the weather was cooler , so
I leaned it too much.

Bascailly the bottom of the rod stretched. The hole where the crank slides into got enlarged.