Answer to some questions:
No I don't trim pistol brass
Wet tumbled with SS pins in a fart
Bullets are from RMR (but I assume are actually armscor they're reselling)
I just ran 300 more through, and I saw the issue some more (though far less, so it's likely a combination of thin brass, slightly larger sizing die, overbelling and overcrimping). Since I want these to be super solid (I'm giving to a buddy, not shooting them myself) I'll probably switch to a different bullet with a cannelure so I can roll crimp, especially since that seems to be the suggested way to load them anyway.
WRT
@Bcwitt roll crimping FMJs with no cannelure- Is this a thing? I thought you can get serious overpressure issues if the crimp cuts into the jacket?
Now to find FMJs or plated with cannelure for less than 20 cents a round...