Export frame rate must match Timeline frame rate.
This started as a question, but I worked out my problem as I wrote it. So now it's a tip, and maybe the more knowledgeable will refine it or explain what is going on.
I've read numerous posts here about out of sync video/audio when you import, but my scenario is a little different My issue is with export, not import alignment.
I have around 30 video clips, from various sources and frame rates (29.97, 30, a couple at 60). They are all from phones.
I have created a 30FPS compilation of videos, and when I export to a format like the built-in Vimeo 1080P, it's fine.
I wanted a very rough draft of the video at 12FPS, to work on the audio in Filmstro. But when I reduce the frame rate to 12, the video is a looong way out. At a guess, the video is 2.5x too slow (i.e 12/30ths). I have changed the project settings to 12FPS too (it was originally 30), but this didn't resolve the problem. I tried constant and variable bit rate on the export.
It turns out, the frame rate of the Composite shot must match the frame rate of the export settings that you use.
Without this, the audio and video don't synchronise.