*Of course* the "weird" [ ... ] || ... construct meant to avoid
the non-zero exit status of the whole thing wasn't employed
where it actually does make the difference!

Thanks ildar@ for hitting and reporting this, as in

  + verbose '/usr/lib64/propagator exists'
  + '[' -n '' ']'
  mki-scripts: .../stage1/scripts.d/80-make-initfs: unable to run script.
  make[3]: *** [run-scripts] Error 1