mkdud actually warns you about this:
# mkdud --force ... rpm-4.*.rpm
[...]
Installation System:
*** Warning: includes /usr/src/packages. ***
/bin/rpm
Without '--force' mkdud will fix the dud to prevent these issues. (The
mentioned directory must not exist in the install environment.)
But unfortunately you need '--force' to build a dud for the currently broken TW
iso. :-(