I don't do a lot of audio work in Premiere, but, this is so simple it really makes me feel less than intelligent!
The latest, v13.1, easy sequence and laying some music down on audio track 3. The music is in stereo from the source monitor, but, mono when played from the timeline, both in sound and vu meters. Checking and rechecking settings it would appear all is set for stereo playback, but, it's not happening.
Does it look like stereo on the timeline? Pr Pro has something they call a stereo track which will show two waveform patterns in the timeline. Or do they call it "dual mono"? I have a stereo track that was imported from FCP onto two mono tracks. You should be able to send the tracks to a submix bus that would play them as stereo, if they are not a stereo track on the timeline. I'm sure there are more buttons and settings that would apply.
Independent/personal/avant-garde cinema, New York City