Damien. yes we Plumbers are always experiencing failures in pipes (copper included).
You stated “fracture” was it stress related?
You stated it started to leak at 14 years but was not discovered for another 3 years.
Are you saying that moisture was noticed at 14 years & a flood at 17 years?
For the past 25 years to my knowledge it has been Industry practice to not put joints under a slab, Prefference is given to conduits & certainly “T’s” are never installed under a slab.
I can remember 40 years ago I installed the plumbing in a slab, aprox 5 years after I was called to a failure, everywhere that the copper pipes were joined & in some cases came in contact with the slab there was a failure, all stress related because the pipes were capturer where they were encased or passed through the slab. The constant movement of the pipes beit expansion because of pressure or movement by the movement of the water within “worked the copper till it became hardened & cracked.
I have never installed pipes that way since. I always bring the pipes through conduits & do the jointing above ground or above the slab.