Reported by @ollystephens via Slack.
Problem was the RPATH sanity check for M4-1.4.19.eb was failing because no RPATH section was found in the m4 binary, despite having the --rpath configuratoin setting enabled. So somehow, the RPATH wrappers were not being picked up...
We should run a quick check to make sure that the RPATH wrappers can actually be used?