the media advance calibration of the printer is slightly off. calibrate it. thickness of materials affects advancement. use a 10x loupe to see the calibration test. dont rely on your eyes.
if calibrated dead on, and still doing it, you may want to switch to a higher output resolution, some colors with moderate black values can do this.
Its common for rips to replace some of the CMY build with K, this is called grey component replacement, or GCR. Its defined in the ICC profile process. If so, spot and rgb colors will convert with that while colors made with a hard CMYK callout will not. This is why the roland vs pms color issue, but it isnt the cause of the print defect I'm seeing here.
Lower platen temps will help ink wetting and flow, but not for the black values you're after..
ink type makes no difference.