See if it's tight to the carrier at the back of the gas section, or at the middle of the bolt.  Either can be relieved by spin sanding.

Also, check the back bugle section of the gas portion on the bolt.  Some are known to not be milled correctly, leaving a high step just before the flair that will conflict/bind with the back of the carrier gas section as that high step tries to enter such.

THe MP testing will not pick up if a bolt has been incorrectly heat treated properly.

When heat treated incorrectly, the metal is too brittle without any type of fracturing for the MP test to pick up, and instead of the metal having some elasticity/give to it to allow shock, it shatters like glass in a shot time instead.

Agreed though,
Get a hold of the supplier for a free replacement since the bolt was defective from the start, and life/use span played no part in it failure of the part

Out the gate, parts are not made by most of the suppliers, but by subcontracts to them. So it may be that the carrier and bolt where not made by the same subcontractor, or even heat treated in the same batch firings.

Next, every suppler (yes even colt) has had a improper batch of heat treated bolts.  Granted that Colt tends to have a better track record when you are talking military issued parts, since not only are the parts tested by the subcontract and the supplier (them) as well, but finally batch lot inspected by the government as well (read if the government fails the lot from there sample test, the entire lot is sent back for a new replacement batch at the cost of the supplier).

So lets take this same example back to the civilian world.  Granted that some bolts have been MP tested, but as stated, MP testing will not tell you if a bolt has been heat treated incorrectly, only if you have a crack in the surface of the metal alone.  Here you need shear testing, and until a supplier/sub contractor does batch lot shear testing, and marks such in a way to prove that is has been done (read letter stamping a heat treated part is not the smartest way to go, and will need to laser engrave), then you have no real idea of if the heat treating was done correctly or not (read if the part is going to shatter or not down the line with a very short service life).

Understand that Mp testing is running a charge through the part and using iron dust to coat the part to check for cracks.

A crack in the metal will disrupt the electrical magnetic  flow across the surface of the part, and the iron powder will collect on both sides of crack predominately to show it's location.
Yes, you could have a machine shop do this for you, or it's something that you could do at home as well.

But to regress, chances are the part was not cracked when produced, but one side cracked before the other due to working stress put on the part through use.  Chances are the black that you see on the cracked section is not parkerizing, but fouling that has collect over time until the part totally failed (other side cracked as well).