I'm getting the exact same behaviour with Streaming Stick 4K and an LG Oled through an Arcam AVR. The problem started with the Roku firmware update at the beginning of this month. Sometime going back to the Netflix Gui and starting playback again will fix it but it is not consistent. I always get the same behaviour when it breaks which is a black screen with Dolby Vision detected and sound working.
The only solution I have found is to force the TV type in Settings to 4K HDR in order to disable Dolby Vision. HDR10 works fine.
I am having a similar issue, but so far only with DV on Hulu and Disney+ that I signed up for at the beginning of the month. Worked fine for about 3 episodes of what we were watching , and then just black screen with sound. Spent some time chatting with Roku support last week, but no fix yet. Paramount and Peacock work fine with the 4K streaming stick to my LG B4 oled, very strange.
