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.
Dan Roku Stick (3600), Ultra (4640), Ultra (4670), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.