I can confirm this is happening to me as well.
I have an LG OLED65CXPUA TV, Roku Ultra 4800X, connected through a Denon AVRS70H receiver.
I'm using the Media Player HDMI input on the Denon which is closest to the HDMI out port as recommended. Plus I'm using a high speed Fiber cable between the Denon Receiver and teh TV, and an 8K rated high speed HDMI cable between the Roku and the Denon.
Everything was working flawlessly with this setup prior to the update to Roku OS 10. Now, it will randomly flash black for a few seconds and then come back and it will display the Dolby Vision logo in the upper right of the screen. The audio continues uninterrupted.
This happens with any app I'm watching through (Youtube TV, Netflix, HBO Max, Amazon Prime Video, etc.) this doesn't happen when I watch through any of the LG TV version of these apps, so this is clearly a Roku issue and seems directly related to something in the 10.0 update.