I recently bought a Hisense 58H6550E with AndroidTV on it (8.0 / Oreo I believe), and I've mostly been using it for the Chromecast functionalities, so I've not logged into it with my Google account, or logged into any of the apps (like Netflix) with any of my information.
This has seemed to work fine -- I can cast YouTube videos from my phone, Hulu videos, etc, and they all cast fine, passing along my account information to the Chromecast flawlessly.
When I try to cast Netflix from my phone, however, I get the message "Both devices must be signed in with the same Netflix account" from the iOS app, a message I can't even seem to Google with any results.
Now granted, the Netflix app on my AndroidTV does prompt me to "Sign in" when I load it up, but I want it to work seamlessly like Youtube / Hulu does. When I cast Netflix to standalone Chromecast devices plugged into another one of my TVs it works this way without me needing to "sign in" on the Chromecast, and there has been times where friends come over and cast Netflix to our TV, so having to log into my account on the TV wouldn't work for those cases anyways.
Is there any way around this? Has anyone else ran into this?
Yeah, this was a deliberate design decision by Netflix. They handle casting to ATV differently than casting to Chromecast. The only real way around it is to sign in to Netflix on your Android TV device, and as you've pointed out, that approach has flaws.
We all hate it, but that's how Netflix rolls.