If it were El Cap, then everyone with El Cap would see it, and there would be a lot of howling on this list. I'm on 10.11.6, and my prints are fine, so the issue is likely not the fault of the OS.
With apologies in advance if these obvious thoughts are all things you have already checked. . . off the top of my head:
Could it be in the image? How does the image histogram look? Is it solid all the way through it, or are there gaps? What is the bit depth of the image? What happens if you print without using QTR? What happens if you test with an unmodified curve (one you didn't linearize)?
Have you run an automatic nozzle check, or just the quick one?
Are all the QTR settings are correct? Mode? only curve 1 used? Highlights = 100; no advanced adjustments; right ink for the paper? Image set to GrayGamma 2.2? Color management off?
All simple suggestions, and again, my apologies if you've double checked these. In my over a decade printing (and teaching it) I'm still known to overlook something basic now and then.
Good luck!
Tracy