I just realized I have a Grand Prix frame hanging in the workshop. The tubes are mitred, but crudely, and the head tube is a one-piece bulge-formed unit. E.g. here's the HT-DT joint:



If the tube on the OP's frame wasn't fully inserted, I think it could lead to the type of crack seen on the lug.

Perhaps the OP could pull the fork on his bike and take a look?