In Nextcloud 32 the login chain was changed, this means to allow login by email you need to adjust your code.
This means checkPassword of the user backend will no longer receive the user id when logged in by email but the email directly.

So to allow to login by email you need to fetch the matching user id of that email manually in your code.
You can have a look at the local backend on how to do this: https://github.com/nextcloud/server/blob/3c4feff028ab87b57a29e5771cde8ba6c5b7e4b0/lib/private/User/Database.php

See also: nextcloud/server#47686