The brass (FC) has seen about five firings but perhaps I should try new brass.
Five hot reloads can expand heads and primer pockets especially with soft brass. It also sounds like you started with the max load which is not a good idea. Could be bad primers, head space, oily bore or case, bolt setback or any number of things in addition to bad primers. R26 is unusual in not showing any traditional pressure signs until you are really high up there. I suspect it is so progressive the maximum pressures are further down the bore and affect the case less than single base or non progressive powders but this is just a guess on my part.