Same here.
I thought with this Ticket (https://gitlab.com/gitlab-org/gitlab-foss/-/issues/28996) the functionality must be given, right?
If I open my Webpage and have a look at the certificate, it only valid vor *.gitlab.io
But in gitlab-settings it looks like that a certificate for the domain was created?!
You can check it at: https://blog.erfolgsbeginn.de/
What can I do to solve this?