disable logon with email
The web interface login dialogue mentions it is possible to log in with either username or email, and the login works, but after that things get weird, and I have to explain again and again people should log on with their username.
The user_ldap plugin has a fix for that, I even found the commit that applies the fix.
https://github.com/nextcloud/server/pull/3698/files
Could you do something similar for your plugin?