lancebrown-ochs , I just opened a voice-over project from the time period when I generated this question. I had an archive of the rendered mp3 from February 2018 to compare to, so I just opened the project file and did a Song>Export Mixdown. Comparing old with new, the bad mp3 output issue is fixed.
This question was raised when S1 was on v3. Now I am using v5. I'm not sure if the mp3 codec changed, and to be honest, since the issue was experienced nearly 3 years ago and I never heard from PreSonus about addressing a fix, I completely stopped using the mp3 option and switched to M4A>AAC. However, I had noticed that sometime in v4, they had added a "Mono" checkbox in the output options, so that was at least an indication that part of my request had been rolled out.
For my A/B comparison, I output a new 128kbps mono mp3 of the timeline and then created an empty project and dropped the old mp3 and new mp3 on tracks. I slipped one to match the timing of the other, then used the Solo button on one, then the other while looping various segments of the audio. The difference is night and day different, so I consider this solved.