We have tis problem as well. Setting the resolution to 4K HDR 30Hz instead of automatic fixes it temporarily, but after some time, the Roku goes back to auto-setting resolution without us asking. As a result, we're having to go to Roku settings daily to set the resolution to be able to use Netflix, and it's quite annoying.
Clearly it's a huge problem, seemingly affecting most if not all Roku devices. Imagine how many people that is? I'm annoyed that after 5 days I've yet to even receive a reply from Roku about this other than one 'are you still having a problem?' auto-reply. But again, imagine what this must be doing to their support staff. The smart thing would be to get a statement out about what's causing it, the temporary fix, and an assurance a new update is in the works. Crickets is NOT an appropriate tactic.
The workaround that seems to be working for me is when in the Netflix channel scroll to the far left and then scroll down to ‘Exit Netflix’. The next time I then select Netflix, it opens fine
Roku is aware of the problem, and I have been told they are working on a fix. If you have a 4K TV, go to Settings > Display type and select either 4K 60Hz TV or 4K 30Hz TV, whichever works better for you. (no HDR) that seems to solve most of the problems.
Let me join the chorus of those having this issue with Netflix. I have a Roku Ultra 2018 model. This problem just started in the last couple of weeks.It does not happen with any other channel. I also have to reboot the Roku