When i using MkDocs, i found url(Custom domain) always change to default(empty). That makes i have to push my domain, each time when i deply. I saw this issue on Docusaurus, too.
This issue makes me bother. I didn’t want to write domain always. So, i found solution.
If you want to configure this problem, you have to make CNAME file on static folder(/static/CNAME).
You can use your domain on CNAME file. You have to do like this. You can solve this problem on github pages.