I noticed that the Jekyll version used GitHub Pages is malconfigured in a way
that the variable site.github.url does not point to the web location of the site itself
but to a non-existent page on GitHub. This page demonstrates this
and also verifies the presence of this bug.
The above 2 links should be exactly the same.
The bug has been fixed!
This bug has been noticed on project AND user pages.
As it turns out this bug is only present if you build your GitHub Pages site locally without any GitHub API key set.