Problem editing video with attached audio on M1 Air - dropping audio

The file is a 1080p MP4 export from Microsoft Stream (approx 1.3GB, H.264, AAC), from a Microsoft Teams call with approx 20-25 participants. The audio was mixed in with some kind of magic from the software, so no action to set up the recording on my part other than pressing Record, though it did take place on my work Windows laptop rather than my Mac. The actual audio stream is mono and fairly lowish in quality, but that's on the source side.

I can re-export from Stream, but I guess I'm kinda stuck with the source file? Should I try running it through another program prior to import?

There is a good point in that I should check into excluding locations with Bitdefender, but that article is dogma as much as practical help.

I've owned Macs for 14 years, this is my third, and I've chosen to have security software more recently due to the growth in Mac malware. Not going to stop that, as keeping fingers crossed doesn't necessarily protect you from zero-day attacks, and it's using the current APIs Apple has put in place specifically for security software (https://developer.apple.com/documentation/endpointsecurity).

I see this in FCP and in my exported video, and only when working with clips in the timeline. Playing from the clip browser or from an unedited version of the clip, I hear the full audio, and the waveform matches up with that.

When editing, parts of the waveform disappear, as does the audio.