It looks petty nasty. Sorry this happened to you. According to what I can see on broken area, it doesn't look like it was overhardened or to sfot, actualy looks pretty diccent.
I heard before that some of the RBs had this problem, but as Pter mentioned it was 11mm crank. It is only one thing I can sugest-try to make pic of broken part closer and put it here. May be I can see if there was initial crack during the manufacturing ( heat treating defect). It happens some times and it is really hard to control, even if use ultrasound QC.
I guess it just not lucky engine, get another cranck.
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