Sorry, I couldn’t reproduce here 
Created several users, with and without main_domain value. All working fine.
Logged in ncadmin account, in https://<domain.tld>settings/admin/additional, what gives switching between 2nd and 3rd choice for SnappyMail (“Attempt to automatically login users…”)?
Also, you can enable debug logs here.
You can also try to reset the “suspicious login” mechanism, but I’m very doubtful it may change something…
cd /mnt/repo-base. scripts/base.shdocker-compose exec mariadb mysql --user=root --password="$MYSQL_ROOT_PASSWORD" $MYSQL_DATABASE_NC -e "delete from oc_bruteforce_attempts;"