Export Group channels VS Audio channels_VOLUME PROBLEM

I’m having an issue when exporting tracks.
The way I have my recording template set is, I have my “main tracks” for recording drums, I then duplicate those tracks (say if I need to record something on top of the main tracks, like cymbals, extra kicks, extra snares ecc) and send both the “main” and duplicates to groups (ie: “KD IN” and “KD IN (D)” go to the “Kick” group, “SN TOP” and “SN TOP (D)” to the “Snare” group and so on) and usually leave all the faders at unity.
Now, when I don’t need to record extras (so I don’t need to export both “main” and “duplicate”, but only the “main”), I export only the audio tracks using the multichannel export, while when I have recorded extras, I export the Group Channels.
The thing is, some of the tracks (for no reason) have LESS volume when exporting the Group channels.
I just recorded 11 songs and 5 of them (the ones which have been exported with the group channels) have less volume in some tracks. Not ALL of the tracks have less volume, but always the same ones from song to song (ie: Room L, Room R).
So what I did was open the projects of the songs whose tracks I exported only the Audio channels, route those audio channels and their duplicates to the usual groups, and then all the songs finally have the same volume.
Is this some kind of bug? Or am I missing something/doing something wrong?

Thanks for your help!

If you send a mono signal to a stereo bus the level on each side depends on the panning position…