How do I troubleshoot screen capture utilities only capturing one side of audio?

The task as asked is in effect "how to sum two inputs to one, stereo to dual mono?" one side being the mic/line etc, the other side being just silence.

If you could persuade QuickTime, or any other recorder to record only mono audio this would be one way. If you need additional music etc that must be in stereo, then somehow you need to persuade the Mac itself to sum both yout input channels to mono.

I cannot seem to find any way to achieve that with the Mac itself, nor much of the commonly-available audio routing tools - Audio Hijack, Loopback, Audio-Midi setup etc, testing with Aggregate inputs & everything I could think of.

All I did manage to achieve was to sum before sending to the Mac. In my case, it needed the input software of my Line 6 interface. Setting the input to only one of the 2 potential channels Line 1, Mic 1 etc, then the software itself was doing the sum to mono for me & the Mac would then be happy to place it in the centre of the stereo image.
Setting the input to Stereo Line/Mic etc would send to just one side or the other.

Presumably, this would be considered 'expected behaviour' for a stereo source, though it does look like if you use the Mac's headphone input or a webcam etc, then it already 'knows' there is a single mic, not stereo.

Interface image, just as an example -

^{I didn't take the time to clean the 'amplifier' setup to be straight clean, so it still looks more like a guitar amp & pedals. That doesn't affect the routing, only the audio quality]}