Ok, some answers at last as to what is going on here:
NAGL used a dichroscope to inspect some raw moissanites and figured out what is going on.
Moissanite is doubly-refractive and thus has effectively a primary and secondary color.
Normally, you mostly see the primary color but in certain lighting, the secondary color can come into play.
If the secondary color is excessively strong, then it can suddenly color modify your stone and thus you get the chameleon or moody like behaviour of moissanite, but one that varies stone to stone.
Therefore, we are going to have to start pre-screening all Mira candidates using the dichroscope to ensure they don't have excessive hidden secondary color and thus avoid this buttery yellow from showing up only under certain lighting.
Here's a link showing a dichroscope in action and you can see how it shows the primary and secondary color in some other doubly refractive material:http://www.faceters.com/askjeff/answer36.shtml
Basically though, this means we'll have yet another hurdle to screen out Mira candidates and since we're already rejecting up to 50%, thats going to make the Mira cut even harder to pass.
But, thanks for the photos and feedback on this thread as it definitely helped us solve what otherwise seemed a bit inexplicable.
Also, we'll contact hync and see about getting that Mira back and sending out a dichroscope screened replacement Mira.