I have had the same issue now with two different Roku devices, a Roku 4 and a Roku Ultra.
I have a pretty simple setup:
LG 4k TV
Roku Ultra 4670X
The receiver has other devices attached to different inputs (Record player, PC, Playstation).
When switching inputs back to the Roku, it will often occur that I will get audio, but no video. The TV indicates no video signal. When I fist got my Roku 4, it did not happen often, but it happened more and more. A factory reset made it happen less, but it got worse again. I replaced the 4 with an Ultra and now it is happening again.
I will say it happens most often when launching the Youtube app when casting from a phone, but switching to the roku input by any means can cause it.
The only fix I have is to reboot the roku, then it works fine for a little while.
My first suspicion is that the HDMI cable isn't rated at 18 Gbps. When I had a Roku 4 with my Samsung TV, it was just fine with 4K material. However, when I replaced it with the Ultra, I immediately had video issues. They were resolved after I replaced the HDMI cable from the Roku to the Yamaha and the Yamaha to the TV with Amazon Basics certified cables. I paid less than $10 for both, and never had a video issue since.
The Ultra supports HDR, which the 4 did not. That alone requires the higher rated bandwidth cables.
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