With the Yamaha did you turn set monitor check to skip in the advanced menu? I am not sure if it will solve this problem but it has solved a few HDMI problems in the past with Yamaha receivers.

I now have the HDM-GA1 gigabit accelerator and 10 meter cables (MHX-HDME10) installed in my "testing lab" in my home. They are connected to the Samsung 2015 model UN65JS9000 and the 4K UHD UBD-K8500/ZA blu ray player. (I also have .5m and 1m "pigtails" connected through keystone HDMI connectors.) As Brett indicated, these cables and the accelerator were necessary to achieve the best picture possible in this scenario.
Fortunately, the Samsung TV will display the following to confirm all is properly connected...and the appropriate HDMI input is selected on the remote: "3840X2160/24PHDMI UHD COLOR UHD". I actually created a trial and error type spreadsheet to tack different cable/input/picture setting scenarios.

I have arrived at the conclusion that, due to a required HDMI UHD COLOR manual setting in the Samsung TV, it is necessary to bypass the 4K Yamaha RXA1050 AVR for video switching. I believe this will be true of any AVR, regardless of the brand, at this time. This HDMI UHD COLOR TV setting is input specific. This is because other non-UHD sources will show noticeable color distortions if they share the same HDMI input when set to UHD COLOR. Until this setting can be switched automatically on future TVs, this will require 2 HDMI cables feeding the TV. One for UHD, the other for non UHD. I still use the HDMI audio output from the UBD-K8500/ZA to the AVR.
Thank God for Universal remotes to do all the dirty work for the client!
I would also like to everyone, especially Brent, for all the input in making this effort successful.
I am still in the testing mode, and will provide any relevant updates. I will be using the same setup later this week with a Samsung 2016 TV, the UN65KS8000FXZA.
Finally..I am personally still very much in the learning mode. Of course, that never stops in this field. I am still wide open for any further input, disagreement, or suggestions.