It is because of the thinner copper of the MHY cable (not the 4K part).
HDMI is Bass Awkards, the display powers the output of the previous device.
The cable box is pulling (demanding really) more current than the MHY cables are passing down the TMDS lines. This is a newer problem and has caused me more than a little aggravation, it also took about a day for me to figure out what the failure was
The correct solution is the less expensive (but larger diameter) MHX cables.
We are seeing the same issue with the DirecTV receivers.