Starting a new thread because although it's the same issue, it's under different circumstances. Mods can merge if they like.
So, I created a new project last night and imported video from my camera. The Media panel reported the video as being an .m2ts file with a framerate of 29.97 and an audio sample rate of 48000.
I changed the project settings to match these numbers exactly, imported a second clip with the same stats, then edited my video and exported it with the Vimeo 1080p preset as an .mp4 file.
Upon creating a new project and importing the newly-exported file, I noticed that the final three frames of the audio track were "hatched out" with diagonal black lines.
Just for completeness' sake, the last shot of the video was an image (imported .png) with no underlying audio, so there's no way you would know it was happening just by watching the exported film.