Even the pretty Grendel bolts can shear

[VASCAR2]

I’d be most concerned about case volume of FF x39 Starline brass with IMR8208XBR and 123 grain bullet. Yours is not the first report I’ve read on this forum where 8208 was the powder in use when a bolt failed. IMR8208XBR reportedly spikes pressures quickly, especially with heavier bullets. It’s entirely possible the steel in the bolt had a flaw and you’ll probably never know. I’m with LR1955 I don’t think the bullets were the problem unless they were jammed into the lands which might contribute to higher pressure.