As confirmed by debug information when setting PYLAUNCH_DEBUG=1, the shebang seems to be ignored when using '#!//usr/bin/env python3' but works fine when using '#!//usr/bin/env python'.

This is with '#!//usr/bin/env python' (proper):
------------------------------------------------
(.venv) PS C:\pytest> .\script.py
launcher build: 32bit
launcher executable: Console
File 'C:\Users\karypid\AppData\Local\py.ini' non-existent
File 'C:\WINDOWS\py.ini' non-existent
Called with command line: "C:\pytest\script.py"
maybe_handle_shebang: read 167 bytes
maybe_handle_shebang: BOM not found, using UTF-8
parse_shebang: found command: python
searching PATH for python executable
Python on path: C:\pytest\.venv\Scripts\python.exe
...


This is with '#!//usr/bin/env python3' (wrong):
------------------------------------------------
(.venv) PS C:\pytest> .\script.py
launcher build: 32bit
launcher executable: Console
File 'C:\Users\karypid\AppData\Local\py.ini' non-existent
File 'C:\WINDOWS\py.ini' non-existent
Called with command line: "C:\pytest\script.py"
maybe_handle_shebang: read 168 bytes
maybe_handle_shebang: BOM not found, using UTF-8
parse_shebang: found command: python3
locating Pythons in 64bit registry
locate_pythons_for_key: unable to open PythonCore key in HKCU
...


As you can see in the second case even though it regognises the python3 command, it makes no attempt to find it in the path.

Note that it appears that Windows installations only carry 'python.exe' and do not have a 'python3.exe' (neither in the native installation folder, nor in the virtual environment folder) so searching on the path would only 'work' if the user has copied python.exe to python3.exe in their <venv>\Scripts folder. (I actually tried that and it did not work).

A proper solution would probably need to search for 'python.exe' even for the '#!//usr/bin/env python3' shebang to detect if a virtual environment is present, possibly even confirming that the virtual environment is of the appropriate version.