Roosters pure for rose comb have a problem with lowered fertility, especially if bred with many hens. Roosters with just one gene for rosecomb still can have a very good looking rosecomb, but have much higher/normal fertility than the rooster pure for rosecomb.
It's this little genetic quirk that helps the single comb gene float around in pretty much all rosecombed breeds. So far only the Wyandotte folks are much more open and informative about it.
It's not necessarily a matter of the bird being mixed, just the stock was not pure for the rosecomb, nothing more. (and it is not a bad thing either, unless trying to breed for pure exhibition stock- many exhibition stock breeders single pair their birds or mate a rooster with few hens, in that case a rooster having low fertility from being pure for rosecomb is not much of a problem- the rooster is breeding the same hens 'more often')