Edit - 11/19/2020: This now occurs sometimes on YouTube ads as well. Don't get it.
I've recently (over the past month or maybe more) run into an issue only when streaming certain things over my Roku Ultra. The device verifies 4K at 60FPS, 4K HDCP 2.2, and 4K HDR at 60FPS.
The issue manifests when watching pretty much anything on Prime Video, and very specific content on YouTube like paid shows, etc. I am able to watch other 4K content on YouTube just fine, Netflix works perfectly, etc. However, when the device attempts to stream this particular content, the screen goes black and I hear random bits of audio every second or two. The only thing that seems to snap the device out of this state is the Home button -- no amount of hitting Back will get me back to the Prime Video home screen or anything else. Sometimes, there is an "HDR" indicator that displays up in the top-right, and sometimes there isn't, so I don't believe it's HDR-related.
I've tried switching cables (each time verifying HDCP 2.2 and whatnot) and switching HDMI inputs on my TV, which is an LG smart TV (with all updates having been up-to-date at the time of this post). This is the only TV I've ever used the device on, and I've had the TV for over a year and probably the Roku the same amount of time. The issue seems to have just cropped up recently.
I also have periodic issues (every few months or so) where the volume button on the remote will turn the TV off or something, but am guessing that's unrelated to this. Just figured I'd mention it.
Thanks to anyone who might stumble upon this post!