With any troubleshooting, I follow a rule of process of elimination or use simplistic methods to verify suspicions.
if it were me, I would try swapping the Apex feed wires with the other driver that isn't causing issues. This will confirm at least which area the problem lies... and we can continue to zoom in after which.
BTW - I'm currently doing some repairs on my DIY light, and it appears that the DIY concept has just fallen off the face of the earth with many articles disappearing and vendors getting out of the market!
Hope you fix yours!
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