I have been experiencing the HDMI Warning on the Stick+ that is in my living room (I have the same model for my second tv). It started with Tubi then it was doing it constantly on Peacock, then last night it was happening when opening AMC+.
It doesn't matter which streaming service you are going to use, as this will happen to any of the streaming apps, and
There is a brief fix to get rid of the HDMI Warning, and
You will have to replace your Roku device within a year (my previous experience).
The fix is simple. You unplug your Roku's power supply. You shut off the tv. Reconnect the Roku's power supply. Turn on your tv. Also, make sure your Roku device (and apps) is updated. From the Home screen, go to Settings>System>System Updates. Once updates is complete, you may want to restart the Roku, just because apps like Peacock don't work well without the restart. You may also want to power cycle your modem/router/gateway to clear up any audio-visual lag.
Unfortunately, the fix is only temporary and you will see the HDMI Warning again, within three to four days. It will just keep happening until you replace the Roku device the warnings are displaying on. My Stick+ is less than 4 years old and it frustratingly keeps happening.