Dave, nice shooting as always. Your gun likes that stouter load for sure.
So were you loading your rifle with a cap on the nipple??? I don't know if I would want to do that even though White's safety system seems pretty foolproof. I wonder if you sanded a little off that nipple then maybe it wouldn't take one hammer strike to set it.
Another possibility is if maybe the spring could be a bit weak. Have you thought of trying the spring from your Bison or your Ultra Mag to see if that made a difference.
Anyway, nice shooting.
Art