I just encountered the same confusion as others in the linked thread. The pipx tool has this in the man page:

       --pip-args PIP_ARGS
              Arbitrary pip arguments to pass directly to pip install/upgrade commands

So of course I had a space after --pip-args. It failed because the software does not work as documented. Someone reported this in 2019. The software is still inconsistent with the docs. Yet someone closed the bug report.

Re-opening or requesting re-opening requires licking Microsoft’s boots. I’m done with MS Github logins.

UPDATE: prime example of someone who thinks inconsistent docs are “not a bug” in a crossposted thread.