I’d say the original had either rusted from the inside of the paint had blistered and the spots you show are corrosion. Coatings generally protect steel but they have the unfortunate side effect that when they fail they concentrate corrosion around the area of failure.
I imagine the frame material was thin and then the blaster has gone straight through. I don’t know what kind of profile* they would be going from but with chilled iron grit you can remove material fast if you are not careful.
I’d get a new frame though as I’d be worried about the rest and it’s going to cost alot to fix it.
*Typically we go for SA2.5 to SA3 50-75µm