Hi Chris,
Thanks for explaining it. Email addresses are used for login by design (with a compatibility extension to let subscribers login with usual names). We will consider changing it and enable generic names (w/o @domain.tld at the end)
BTW, you said email-like login format breaks your workflows. Does it mean these users access some other system of yours too and have different login there? Would a feature of auto-login into Plesk help here? If you have their Plesk password, we can help with a simple web form to submit...
Regards