If I re-encode it as AVI at 25 fps - it syncs perfectly.
Given it happens with mp4 footage straight out of the camera AND files re-encoded as mp4 - it seems like something funky can happen with mp4's
I also changed the encoding setting for audio to "constant bitrate" - from "variable bitrate" - which may or may not have something to do with it.
So my part of the problem is solved.
PS A web search for "DAW video sync problems" shows that this can happen in several DAWS and has been a problem for a while. It seems somehow connected with how DAWs handle bit rates and sample rates in audio that differ from the DAWs own settings.
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