Are those stock heads? I have seen this several times and every time the root cause was a machining problem. Being it is the first (or last bore) bore in the machining process I would look there. The tool could have had a chip or burr and caused an oversized condition of the pocket.

 

Your car probably set a code when the valve seat dropped. You should look at your freeze frame data and you will know what temperature your engine was at when it failed.

 

any updates??