I'm not sure I understand the break. I'm seeing one half of the master link still on the chain, intact, and the other half loose above it, intact. I see part of a broken link on the chain; Where is the other half of that? If that's on the underside of the pin on the loose link, I still don't see how it all went together, and I use master links. I see two completely intact outer plates of the master link.

Beyond that, not surprising to fail at master link, each pin is only press fit (or welded) into one side so will not stay as straight, and bending force on that pin (instead of pure shear) will bend and fatigue the outer plates on the master link, and also adjacent link as shown. Replace link.

EDIT: The thinner side plates of 12s with higher strength material, does not change the lower lateral STIFFNESS of a thinner plate (young's modulus does not change with strength). That is a stiffness related failure, IMO.