But this was also a misunderstanding from my side. Since "pristine-tar" also refers to a Git branch when maintaining package sources with Git, I thought you were referring to the vanilla upstream source.
My bad, sorry! I was really referring to the Debian tool "pristine-tar" for managing pristine-tar branches. The reason I stumbled upon the problem in the first place was that the latest version was mysteriously absent from the repos, which led to discovering this: https://buildd.debian.org/status/package.php?p=pristine-tar#problem-2 And that in turn is caused by xdelta3.

