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.