This behavior is perfectly normal. When you set the Bus to Post Fader and you pan your Mono audio channel to the left, and your Mono Aux to the right, then the Aux level will drop a few dB because it's not getting the full signal. See Pic 1 - Mono - Mono (Note in the pic that we get the exact same results in both Logic Pro X and Logic Pro 9.)
If you change your Aux Input to Stereo, and pan your Audio Channel to the left and your Aux to the right, than you won't get any signal at all in your Aux (See pic 2 - Mono - Stereo).
If you don't want the Aux signal to be lower when your Audio channel is panned to the left, you could change the Aux input to receive the Left side only. (See pic 3 - Mono Stereo). Or use Pre Fader instead...
The Pan Law doesn't have anything to do with this. If you set the pan law to 0 dB, the level will seem a little lower when you pan it to the side but the meter will show the same value. If you set it to -3dB compensated, the level will be 3 dB lower when dead center, and the value will go up 3 dB when panned left. A setting of -3dB will reduce the meter as you move the pan to the center position.
Clear as mud? I hope it makes sense somehow. :)
[url=http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/files/mpv_43286_1687_Pic_1-Mono-Mono.png] [img]http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/thumbnails/mpv_43286_1687_Pic_1-Mono-Mono.png[/img] [/url]
[url=http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/files/mpv_43286_1687_Pic_3-Mono-Left.png] [img]http://www.macprovideo.com/forum/upload/upload/server/php/savedfiles/thumbnails/mpv_43286_1687_Pic_3-Mono-Left.png[/img] [/url]