Jason just tested a 75' Key Digital HDMI cable with a 4K Blu-ray from the Panny player - works fine. Just FYI. We sell these.
But did you try 4K60 HDR output?
Either from the Billy Lynn or Gemini Man UHD discs, or HDR from Netflix or Amazon Video where the Panasonic player will only output 4K60? Netflix especially since it will only output 4K60 4:2:2 12-bit BT2020 HDR. That is a 17.8 Gbps signal compared to 8.9 Gbps for 4K24 4:2:2 and 13.4 Gbps for 4K24 4:4:4.