The only time I've seen this happen is when HDR content is being played. If that is the case for you, the flash to black is your TV, not the Roku player. The TV is switching to HDR mode, during which they blank the screen. Now if it's happening with non-HDR content, then that's a different matter.
Your TV is more than likely not the issue, the Roku Ultra firmware is I would bet money on.
No, I don't work for Roku. I have no idea if they are working on a fix, I just keep posting about it on here till hopefully they get sick of seeing the same problem being reported and they fix it.