Apple Mail is confused when the username of an Apple-ID account (iCloud Mail) is the same as a Gmail address of my other mailbox

So, I am using my Gmail-address as the eMail and Login for my Apple-ID:

Apple-ID: eMail (and login) is 123@abc.name (hosted on gmail), Alias is 123@icloud.com

GMail: 123@abc.name

In Apple Mail I am using 2 mailboxes, one for iCloud for getting mails to 123@icloud.com and one with the Gmail account (123@abc.name).

And this is confusing Apple Mail completely... as long as both mailbox are active, Apple Mail permanently tries to send mails of the Gmail account through the iCloud SMTP server. Regardless of what is configured!! I even tried to set specific SMTP hosts for both accounts, no luck...

Also, Apple Mail is permanently using the (and only that one) signature of the iCloud account, I cannot change it, can't even select other signatures.

Only when I deactivate the iCloud Account, everything works as expected...

This worked some time back, pretty sure, but I can't tell when it broke... Funny enough, on iPadOS it works as expected.

It is like the same eMail for logging-into iCloud and the Gmail account is confusing Apple Mail.

[Edited by Moderator]