MIGHT be happening with pied. The leopard might be a new allele of the pied gene that is fully co-dominant (as opposed to the old pied allele that is officially recessive but with frequent but less extreme co-dominant tendencies).
Russ, I think it's like you said, even the homozygous hidden seems to remain hidden. Would love to know what sort of mutation that could be that only shows up when paired with other mutations of the same gene. I'm sure with all the hidden X hidden pairings RDR did early on he produced some homozygous hiddens but then later he did a platy X platy pairing that produced a normal that per the allele theory of platy should be homozygous hidden. Don't know if he kept it and bred it to a homozygous lesser BEL to confirm that pairing produces 100% platy as expected.