even though the alternator is charging. The first car I owned with an alternator rather than a generator (many years ago) was a '66 Porsche 911, and I ran it that way for 4 years and 50,000 miles before I became aware that it wasn't supposed to be that way (by reading Porsche Panorama).

If you take a car with a really dead battery and then jump start it and expect the alterator to fully charge the battery while operating all the other electical stuff, that can sometimes ruin diodes thus.