It's difficult to say without more information. What software is being used and what is the source? I assume the HDMI cable connected to the HDMI 2.0 input on the projector. That's pretty high end hardware. I assume it is used for gaming as well as watching video. Does 4K work with games?
The cause may be something out of your control. Unless a PC has a trusted platform module, it is considered to be an untrusted platform and various copy protection schemes kick in with copy protected material. For example, I find that audio is always restricted to DD2.0 with streamed video sources. Despite the hardware capabilities of a PC, streamed video and audio is not as good as some so called trusted devices such as a Chromecast.
The first thing I would try is taking the RX-A3060 out of the equation and try some different 4K sources. A 4K clip without copy protection would be a good baseline for troubleshooting.