can you take another picture of the fracture surface?
from this view I see a small void (dark empty pocket) in the left surface.

Just a cheap squeeze cast part pushed to it's limits. Could just be bad luck and got a gas bubble in there during casting.
I dont have the parts, only the pictures sent to me.. sent to the owner by the race team who look after his car.
all stock parts it turns out.. Amazed at this.
CNC ported head here cost >£1000 and then built up with standard valves on a race car, revved to 8K rpm? makes no sense to me nor the owner who thought he had uprated parts.
I fear the head is scrap with the bucket bore damage