The point has been made at this forum several time that you may only use listed / Chapter3 wires in a raceway. Does anyone recall the code reference that was cited?

A box is a box, and not a 'raceway,' thus the issue does not arise for pigtails in boxes.

There's also a 'listing and labelling' issue. UL requires conductors (wires) to be marked every so often with certain information, including the NEC abbreviation for the type of insulation.

MC is listed as a 'cable,' and is evaluated as a whole. While UL allows a variety of standard insulation types to be used within the MC, the only marking required is that on the outer jacket of the MC. There's no requirement to mark the individual conductors.

While, as far as I know, all MC is made using ordinary building wire (such as THHN), it's not required that the insulation be identical in form and thickness to building wire. One can speculate that unlike 'ordinary' wire, thinner, or alternative, insulation being present. After all, the individual wires will never be exposed to sunlight or pulled through pipes.

Without the marking we don't KNOW the wire meets Chapter 3 requirements ... so we're technically not allowed to run it through a raceway.

The same issue applies to NM.

That's why I question how the fittings can be used in a code-compliant manner.