Each individual input on the M-Track interface captures audio in mono. Therefore, connecting a stereo audio source to a single input will not produce a stereo recording.
To capture stereo audio, connect the left and right feeds from your stereo audio source to inputs 1 + 2 on the M-Track interface.
Please also ensure that the channels which are routed to inputs 1+2 in your DAW software are panned left and right. This creates a wide and spatial audio image for the best listening experience. note: Many DAWs will have the option to assign a Mono input (1 or 2) to a track, or Stereo input (1 +2), in this case, simply choose Stereo Input, 1+2 and no additional panning is needed.