The problem ❔
I am maintaining the website of the McCann Lab that is currently hosted on the GitHub Pages. Since I set up the custom domain, I was experiencing a weird issue: after every single deployment even on GitHub Pages (originally done by Travis, now by using a dedicated GitHub Actions workflow) the custom domain was removed and I had to rewrite it every time (see ⬇️) which was pretty annoying.

The solution 🏅
Truth be told, it was not difficult to find the solution! It was already
reported several times (including this issue on GitHub
gh-pages/issues/213).
Basically, one needs to include a CNAME file with the custom domain name
and for Hugo (the static site generator I use for this
website), this file should be added in the static
folder. This is actually
well-explained in the
documentation, so once again, read the doc!