I had a similar experience with LG. You need a developer account in order to install applications on webOS TVs. I successfully registered on the LG Developer portal with something like myname+us.lgaccount.com+2021-07-01@mydomain.com, but I couldn't log in on the TV with that email address. Had to register again without the plus signs.
I had something worse(?) happen with one of my accounts for a hotel chain. I entered name+hotel@majoremail.com and they just stripped out the + leaving me with an account that I couldn't use unless I made another account at the email provider.
