Failed plug

Be a little careful - they might "accidently" lose it!

These longitudinal crack-like defects are called sulfur stringers and
sometimes we found them in poor quality resulfurized free machining
steels. It makes the steel very brittle - especially when the
temperature is elevated, and that's why they machine so easily.

I'm not a metallurgist - which is what you really need for a valid
opinion.

If this is what has happened though, as a mechanical engineer I am
appalled - from both a design and especially from a quality monitoring
standpoint.