When I playback a -16 dBFS tone from a mono channel panned center on my DAW, the stereo master volume level reads just below -16 dBFS. But when I pan the mono channel hard left ot right, the level for the master is the same. Now, when I bounce from stereo to mono, my DAW gives me he option to attenuate by 3.5 dB.
I'm aware of the difference between dBFS and dB, and level readings and gain readings, but I'm still a bit confused. What am I missing?

I can't tell for dBFS but for the mono with eventual attenuation of -3.5 dB it's interesting.

When you sum up the L-R channels, you have a +6 dB in level increase, so to keep level constant you have to apply -6 db attenuation. It's a constant-voltage level law.

Now, when you listen to the speakers, the summing is acoustic not electric. The perceived level is only +3 db higher than it should, so to keep the loudness level equal acoustically it's only a -3 dB attenuation you have to apply, constant-power law. It's what the pan pot does in the middle.

Now, you have either -3 or -6 dB so what? The compromise has been usually to have a -4.5 dB attenuation on analogue boards. But in DAW you can have multiple choice to suit the job and the peculiar use in the end., from -3 down to -6 with values on the way.

Thx for both question and answer