[Sholander]

To find the ESN on the NVidia Shield, open Settings then go to Device Preferences, then About and scroll down until you see "Netflix ESN". Yes, you do need to input the ENTIRE string of characters- what was in my add-on before was just the first few parts ending with "ANDROID=TV" I think, and I added all the numbers and letters afterwards. After this I got 4K back without restarting anything.

GREATLY appreciate these add-ons as I cannot stand having the regular apps put junk on the screen, telling me to skip parts and cut off credits. I watch everything from beginning to end, so that is just insulting.

A week ago when it was SD 540 for the first time, this method helped. Three days ago SD 540 again. Several times I reentered the full ESN on the NVidia Shield, nothing helps. No 4k

Have you tried to login to your NF account on a computer, then in account settings check the new option "Manage Access and Devices". There you can "Sign Out" any blocked device. Maybe before doing that it's worth to try uninstalling your Netflix Android TV App first, then turn off your NVidia Shield...
After Signing Out all NVidia Shield entries, maybe need to wait 24h to sign in with NVidia Shield and reinstall Netflix Android TV App.
This is just guessing on my part, as I haven't tried this procedure, yet

I will try this "procedure" on my FireTV4K, but will not reinstall official NF app. Instead I'll use NF addon in Kodi where I can easily slightly change last ESN characters. Just to see if it will last...