Very likely too low in amino acids and other nutrients to support a growing dual purpose bird. Surprising this is the first to show symptoms. Do you know the totals on your nutrient values with the amount that you are mixing them together? Chicks should not ever have layer or oyster shell added in.Has been with other chicks for about a month. Maybe a little longer. I give all my chicken 7 grain scratch, some meatbird every now and then, oyster shells, and cleaned wheat grain mixed in also. And some later pellets, because it's all mixed together.
It takes at least 3 weeks from exposure for Mareks to develop and often presents as limping. Here is one resource that has all the information...
Marek's Virus FAQ
The still eating well indicates it isn't likely coccidiosis adding to the issue. Also no pasted vent or slow or blocked crop. That's a lot of major things ruled out. Do they have access to any form of grit?

