Alternator- Demagnetized field due to prolonged storage

A coworker of mine was installing an aftermarket alternator on his 98/99 Mercedes C230 due to the original one finally calling it quits after 220k of hard working miles.
After installation, we found the alternator (aftermarket) was not charging. Voltage output was at battery source voltage regardless of loads being on or off- gear selection in PRND.

I decided to use scannerdanner's method of checking the field wire with an incandescent test light. As soon as I touched the field wire I saw a light spark and simultaneously the tone of the alternator changed. With the test light removed, we immediately noticed the voltage output on the DVOM, voila, there she was running again. The voltage we wanted to see all along- 13.6 Volts. Scannerdanner is a magician!

We turned it off, started it back up, drove it for a few miles and she was charging normally.

So this really had me confused, I have been doing some research as to how this could happen. Thanks to the internet I may have found my answer, maybe. I wanted to share this with you guys and see if you have ever ran into this problem. In the 10+ years in the automotive profession this is the first for me.

Back in the days of DC dynamos re-polarising the field coils was a standard part of the instructions included with replacement dynamos, though it was rarely if ever required. I have never heard of in relation to alternators before.