The first track I play is an MP3 file that is playing correctly without any issue. The second track I play is an AAC/M4A file purchased from iTunes, and you can see the volume fluctuations in the waveform (as well as hear the fluctuating). Edit: so after a bunch of troubleshooting with Microsoft Support via DM on Twitter, they recommended that I submit the issue to them via the Feedback Hub that's apparently built into Windows:
Hide player controls
Hide resume playing