N12:Export mono track result lower 15dB

Hi everyone,
Few mono tracks, outputs are 5.1, no any group or fx tracks, just directly point to the 5.1 outputs. Need to delievery to other DAW so I choose the tracks itself as export source, choose “mono downmix” on File Format section. Results the clips are lower about 15dB, So I need to change the outputs to the stereo to keep same volume.
Is there any step I’m missing or it’s a must step to change to stereo output.
Thank you all.