Mono clip in a stereo multitrack session is getting converted back to mono on export

I have a Stereo Multitrack session with 3 tracks that should all be playing stereo. When I export the entire session (or just timeline selection), Track 1 is only heard in the left channel, whereas Tracks 2 and 3 are properly heard in both L and R.

What am I doing wrong or is this a bug?

Track 1 is a stereo track that contains edited clips of the left channel only of a stereo source

Track 2 is a stereo track that contains a stereo music track

Track 3 is a stereo track that contains a mono voice-over clip that was recorded in Audition.

When I play the session on the timeline, all three tracks mix to stereo and can be heard in both L and R speakers. Despite the clips on Track 1 being from a stereo source, I have only dragged the left channel from that source onto the timeline. So with no other changes to the track, the clips on Track 1 all play in both the L and R speaker equally.

All three tracks are panned to the center (default). I do have some insert effects on the tracks, but even if I remove them, it still exports the first track to the L channel only.

What am I missing?

Thanks in advance!