User interface text language detection doesn't work

Hi,
We need to add additional language to existing website.

1) choose "Detection method" as "Domain" ("URL language detection configuration" page).
2) When editing default language in "Edit language" page leave "Path prefix language code" & "Language domain" fields empty.
3) When editing additional language in "Edit language" leave "Path prefix language code" empty while in "Language domain" enter "my-additional-domain.com".

"my-additional-domain.com" is aliased in the web-server to the "default-domain.com".

Now, when going to "my-additional-domain.com", additional language is NOT detected. the website displays the default language.
Thanks.