I don't have a lot of time right now, so some detail will be missing, for now anyway. I'll just make some short comments.
Dye Depth.
You are seeing the voltage drop somewhat after some anodizing time, but it isn't PAR. PAR won't work if you are using titanium racking. I went into this in detail in one of he stickys (above). You probably haven't a clue at what current density the work was done at, I couldn't even guess, even if you provided current and voltage measurements. I'll bet however that its lower than you think it is. Use the 720 Rule.
4 minutes dyeing time for anything other than a pastel sounds way too short. The 2011 is probably suffering from excess dissolution (oversize pores). You will notice this in time, the coating will be mechanically weak and prone to fading. 30 minutes is a more reasonable dyeing time with the correct pore size. Your dyeing problem is with the 2011, not the 6061.
Could it just be a dye mixing/contamination problem? Maybe, you've had one before
Small white/silver spots; poor agitation is a likely cause. Could the aluminum have been bead blasted with media contaminated with iron or steel particles? These become imbedded in the aluminum, you can't remove them with anything less than aggressive sanding or surface machining.
Small black spots; poorly mixed dye (like you surmise) is likely, just stirring it regularly during dyeing works for me.
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