While it hasn't been cleaned properly at all there's nothing there I can see that would cause a stud to snap, the raised area appears to be cracked and raised alloy not a lump of gasket
the head should be repairable easy enough I'd of thought

The only scenario I can think of is the thread hole needed tapping out, but instead had a bolt just wound in and it possible picked up on the thread and broke that lump out on tightening it up, if that happened the garage is technically at fault, unless it happened when the stud was removed in which case it's just one of those things