Why does the Apple TV 4K want to retest Dolby Vision when I select a new EDID?
The Apple TV 4K keeps a memory of successful EDIDs. When you configure a new EDID it asks to retest Dolby Vision and/or HDR. The next time you select that same tested EDID it will not ask again.
Netflix on Apple TV 4K only shows 5.1 titles?
The app menu may not say Atmos at first, for example showing 5.1 in Netflix, but as long as Atmos appears under audio settings, start playing content you know supports Atmos and the app will adjust. Prime Video behaves similarly.
I cannot get Atmos from the Apple TV 4K but I can from other sources.
Make sure the Apple TV 4K audio setting Change Format is turned off. If it was switched on to get at least Dolby Digital 5.1 out of the TV before, turn it off and the Apple TV will detect the DR HDMI 8K as Atmos capable.
I get Atmos from every app except Netflix, on both Apple TV 4K and Shield.
Open your Netflix account settings on a computer, check the audio playback setting, make sure it is set to high rather than anything lower, then restart the source.
The wider workflow for Apple TV 4K, including what to change and in what order, is covered in the Apple TV 4K EDID guide.