Rather than explain it via text, I think a video would be better. Watch that here: (password is wca)
My tracks are all mono, my master track is mono, my loudness meter is set to monitor in mono and it is the last item on the master track.
My LUFS meter, be it Insight2 or YouLean or WLM (I've tried all three), reflects -16LUFS on the nose.
The resulting audio file, however, is around -20LUFS.
If I were going from stereo mix to mono output, I would understand where the loss is coming from, but I'm not so I don't.
On the export screen I export to WAV, export as mono.
To solve this I change everything to Stereo and bounce to mono with my meter reading -14LUFS... but that's not what I want to have to do.
S1 5.x Pro (latest)
Mac OSX (Big Sur, Apple silicon)