A couple of years ago I had to replace my rear subframe because one of the mounting lugs had snapped. Like the previous pictures, a groove had been worn in that went through almost the whole thickness of the metal, then it just failed. A previous owner had fitted powerflex bushes to it presumably as a futile attempt to save the subframe.
I can confirm with 100% certainty that a good subframe does not have that groove. The one I fitted to replace it had perfect flat lugs.
It is not the end of the world. Assuming the lug has not failed, just clean the corrosion out of the groove and fill with some aluminium weld, then grind it flat again.
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