I had the some problem when using mypy. What does reproduce the issue is the following:

* Create *two* venv environments
* Install mypy in one of them
* Activate the other
* Run the subprocess call in the activated venv

I then get a message about not being able to find mypy:

.../some-env/bin/python3: No module named mypy.dmypy

I don't get the message when I call subprocess outside of a venv.

I haven't checked yet if my patch fixes this issue, that will have to wait for later this week.