Ah thanks ! I also reported it as #12084.

Ah, that's also visible in the autobuilds. We need to look at them more :-(

@MarcelRaad: Can we make that test fail if it detects a distcheck issue?

I'm not familiar with the packaging process, unfortunately. Probably @bagder can answer that? We have that issue quite regularly, so that would be great.

Unless I'm missing something, we might miss a make distcheck
in the distcheck GHA. It needs to be an out-of-tree build to catch
this (and cover dot files, which it probably does?). This needs to
be tested in practice. Existing tests try to build from a freshly made
source tarball, but one of them is CMake, which doesn't run checksrc
on its own to catch this particular case, the other is autotools, which
is likely not a debug build to break on this either.

I'm testing this in #12088.