Thank you, Steve, for your rapid response and explanation!

I would like to have my installation fully isolated in case somebody (running the machine) fiddles with the installation in the standard location. I used to do that without problems with the 2.7 .msi installer, but now I’ve finally almost finished converting to 3.8 (I know I’m late) and that approach doesn’t work any longer.

I tried to reverse engineer it a bit by looking at the log files and zapping the registry entry HKLM\Software\Python\PythonCore\3.8 (for an all-users install), with no success. Modify or repair would randomly install some stuff in the target dir and other stuff into a previous installation. The Windows Installer technology seems to be rather implicit than explicit. How does it detect the previous installation’s location?

Although it’s not a bug and an /isolated mode doesn’t seem to be feasible, perhaps it could check and warn that /targetdir is being ignored. The doc could mention that it’s intended only for a single installation per version per machine.  

And thank you again for your contributions to make Python on Windows such a joy – highly appreciated! 
Martin