Are these new rear shackles "defective"?

Looks like a parting fracture that wasn’t cleaned up. Probably something to do with the tooling used to size the pins. Appears to be excess on the end of the pin, so, it shouldn’t hurt anything if that’s what it is.

If you’re concerned about it grinding it off and putting a spot weld between the pin and the arm would solve it. Probably overkill, but that isn’t something you want coming apart.