I have just found that when I do C:\Program Files (x86)\checkmk\service>check_mk_agent.exe check -self a line appears when the plugin works:
A Module changes command line of the plugin ‘C:\ProgramData\checkmk\agent\modules\python-3.8.venv\Scripts\python.exe C:\ProgramData\checkmk\agent\plugins\mk_logwatch.checkmk.py’
But it doesn’t appear on my other machine. I assume it’s because MK didn’t install with baking the python-3.8 module, but it is installed. I ran the post postinstall.cmd after extracting the files from the zip.