Sometimes breeders combine a Merle with a Tri and the results are homozygous merle puppies (lethal whites). Since everything was done correctly, this would appear to be impossible. The cause of this irregularity is the phantom merle.
The following information is taken from an article by the Australian Shepherd Health & Genetic Institute. You can find the entire article at ASGHI Cryptic Merles
"Phantom merles, more properly called cryptic merles, are dogs which carry a merle gene but are phenotypically (look like) tri or bi. Most such dogs will have some small area of merling somewhere. One of the most famous in this breed, Fieldmaster of Flintridge, looked like a black tri except for one or two very small merle areas. Theoretically, it is possible that a dog would have no merling and be a cryptic merle, but this would be extremely unusual. Sometimes the only merle spot will be on the tail which is docked in Australian Shepherds. Cryptic merles should be registered as merles because they will breed like merles. If bred to a non-merle, merle puppies will be produced. If bred to another merle, homozygous merle puppies will probably be produced. Merle offspring of a cryptic merle will virtually always be "regular" merles, not cryptic merles.
Excessive white markings in puppies from a tri-to-merle cross are not "proof" that the tri parent is a "phantom merle." Excessive white markings in such a cross are the result of genes which code for white trim and have nothing to do with merle."