Glow plug element is pretty soft and hard to believe it could do that, unless it was an idle bar that broke loose. Others have made similar claims about the element but I've always taken it with a gain of salt.

New engine or not, something beat up the inside of the cylinder head and piston.

I'd say get the crank out of it and get your finger in the the rear bearing and rotate it. You will be able to feel it if its rough. Better if you can get most of the oil out of the bearing with some solvent. Roughness really shows up with a cleaned out bearing. Any little metal particles will make their presence known by the feel. If you can't find anything, clean it up and run it.

Anyway, that's my best guess.