Your failing test case with 3.7 works for me.

If you don't use activate.bat, but just run the venv's python directly, what do you see? I get:

>py -m venv fooenv

>fooenv\Scripts\python -V
Python 3.7.0

>fooenv\Scripts\python -q
>>> import subprocess
>>> subprocess.check_output(['python', '-c', 'import sys; print(sys.executable)'])
b'C:\\Users\\XXXXXXX\\fooenv\\Scripts\\python.exe\r\n'

What shell are you using? Above is with cmd.exe.

If you "echo %PATH%" after activate.bat, what do you see?

Before running activate.bat, do you have a python.exe in your path? If so, is it the one that subprocess is reporting?