The round that chambered hard made the bolt close to a different position than the other rounds did. Such has been the cause of shots going wild for decades.
You're one of a very few reloaders to link that flier to how the bolt closed on the round that shot it. Same thing is common with benchresters neck only sizing their fired cases and one has got too long at the shoulder making the bolt close hard on it.
Keep doing all that right stuff shooting tiny groups.
US Navy Distinguished Marksman Badge 153
Former US Navy & Palma Rifle Team Member
NRA High Power Master & Long Range High Master
NRA Smallbore Prone Master