Creating a venv from the python.exe from another venv does not work since 3.7.2 on Windows. This is probably related to the change

    bpo-34977: venv on Windows will now use a python.exe redirector rather than copying the actual binaries from the base environment.

For example running 

    c:\users\$USER\.local\pipx\venvs\pipx-app\scripts\python.exe -m venv C:\Users\$USER\.local\pipx\venvs\tox
    C:\Users\$USER\.local\pipx\venvs\tox\Scripts\python.exe -m pip install --upgrade pip

results in pip installing to venvs\pipx-app and not in venvs\tox. 

Downstream bugreport in pipx is https://github.com/pipxproject/pipx-app/issues/81.