I think the original poster has left, but I will answer briefly if I can
First, thank you for the kind words everyone who mentioned my breedings. That means a lot to me!!
Generally, all of the herding type breeds were about the same and crossed back and forth a ton. Over a hundred years ago, each regional "breed club" created their "breed standard" and tried to breed towards that type - basically what we know as the German Shepherd, Dutch Shepherd and Belgian Shepherds. Each country's breed club closed their stud books at different times. For Dutch Shepherds, that was in the 1970s. Up until then, if the dog was evaluated by more than one judge that it met the breed standard, then it was accepted into the stud book. Meeting the breed standard is simply an evaluation based on physical appearance - which a fawn/brindle breeding could produce. So the FCI dogs coming from the closed stud books have known pure generations back to the 70s - not before.
EXCEPT - there was also the careful introduction of a few breedings to increase genetic population. This was done with dogs that were not 100% FCI. Also, it is known that some overseas FCI breeders misrepresented their papers to make registered working dogs. Lastly, every country has a different standard for getting a dog into their national breed club, and therefore can become FCI registered. There are still some countries that are more or less lax about accepting Dutch Shepherds - so then by default they have FCI registrations.
So where does that leave us? LOL. Heck if I know.

Except to say that every Dutch Shepherd has some outcrossed breedings in there - officially or not. And the only purity standard we could even shoot for only goes back about 40 years.
The statements about KNPV vs FCI are true enough. Good dogs are bred based on work, not physical characteristics. However, there are plenty of breeders with KNPV stock who are working to stay true to a brindle standanrd.
Genetically, the K lokus determines fawn, brindle or black (in Dutch and mals anyway). If the dog has any brindle at all, it is considered brindle - even though people use descriptive terms like "black brindle" or "reverse brindle." Those are just generic terms to describe what the dog looks like. Black is most dominant and represented by Kb, brindle is next dominant and is represented by Kbr, and fawn (yellow) is least dominant and represented by Ky.
A black dog could be Kb/Kb, Kb/Kbr, or Kb/Ky. If it carries black at all, it will be black in color but could carry brindle or fawn.
A brindle dog could be Kbr/Kbr or Kbr/Ky. So brindle dogs can carry fawn - this is common because it is easily masked for generations. The most recent litter of FCI registered long-haired dogs in this country had a fawn. So they still would be FCI registered Dutch Shepherd. It just is considered a fault and not a breedable dog. UKC has a different standard as they say brindle is the ONLY acceptable color, but UKC should really confer to FCI standard....and they probably will.
Finally, a fawn can only be Ky/Ky and therefor can never through anything but fawn.
Sable is a separate term that refers to patterning over the sable base coat. It effects what you see off of the K locus, but does not change the genetic base color.
So in this particular situation - if one parent was a solid black dog, then it would have had obvious outside influence in the lines because it can not carry black without expressing it. But the presence of a fawn/sable puppy is not evidence that it is not a "pure bred" because that color can be masked. If both parents are truly brindle, then the pedigrees deserve further looking into.
Does that make any sense?? LOL.