You may be a victim of a convenience. The latest Audacity convert to MP3 also mixes down to mono. It’s a little checkmark in the dialog and it defaults to checked.
Any conversion of your stereo show to mono will reveal this problem. File format is irrelevant.
The Left and Right channels of some of your work are put of phase.
If you magnify the damaged tracks, you may see the up and down motion of the waves are exactly out of step.
Pay attention who which parts of the show are like that. Music? Only your voice? Something else?
Open a damaged stereo track. Use the drop-down menu on the left > Split Stereo Track. Select the top track > Effect > Invert. Select the top track > Menu on the left > Make Stereo Track. Done. If you magnify the waves again, most of the waves will be going the same direction. This track is fixed.
You can do that to all the damaged tracks and then mix the show again.
Can I predict the past? Did you use a vocal microphone adapter like this one?
https://forum.audacityteam.org/t/devils-adapter/46592/1
That one is famous for creating “magic” voice tracks like this.
Koz