Hi a1ex, I have no idea how to determine whether the meters are accurately recording volume so I don't think I can be of help there (I have no known reference). It may be fine.
I can easily demonstrate that the audio meters do not engage until you hit record or enter the ML menus and leave, playback a video, etc. I can repeat it each time I turn my camera off to on. After you do that you can see levels while non-recording which is what I want to do (want to nail settings before starting to record an interview for example).
The Canon audio menus are set to manual, levels 6 notches or so above 0, and ML menus have audio override on and levels +24. To keep things simple I'm just using the built-in mic. Latest 600D build of ML installed 2017-10-04.
Is that how the camera is supposed to work or a bug, and what sort of documentation is useful (a video of ML's behavior?)
The link to bug reporting is too general for me to do much with. Of course specificity is good and repeatability moves the burden from the programmer to the user.