When using a private domain for a project should the default gitlab domain still be usable?

See https://docs.gitlab.com/ee/user/project/pages/getting_started_part_two.html#urls-and-baseurls
I altered my SSG to use my project website base url. Everything worked. Then I set up a custom domain.

I found that the SSG did not provide the correct path for a base url, it was still adding in the project name to every link. I then removed the base path from the SSG and got the custom domain working.

My question is, is expected that both the custom domain and the gitlab provided project domain should work simultaneously? And if so how?