@oli700 I took a look at a quick sample of some of my brass and I don't get the same tiny dent at the mouth. And that is with several different AR's over the years, some of which weren't even fired from one of my guns I'm sure.
Occasionally I have a side wall dented, but not at the mouth. And the side wall dents are more likely from it hitting the receiver around/near the ejection port somewhere, and probably where yours is hitting too rather than the deflector itself.
And for giggles, the occasional receiver dent
Next time you go out, shoot 10 times with each bolt, collect the brass and look at it and then do the same for the other bolt.
I wonder if the ejector tension is stronger with the new bolt, or perhaps the ejector itself may be ever so slightly longer, which would cause it turn sideways in the bolt faster (earlier) as it is being ejected if that is the only change.
But also having an extractor that holds tighter is also going to change the way that it ejects some too if the claw is not wanting to let it go (more tension in one than the other).
I don't think it would have anything much to do with the gas system unless one was grossly lighter/heavier than the other bolt, but I don't see that being so different that it would cause that.