On FCP 7.0.3 and still having this problem!
I’ve worked out a fix… it’s very clumsy but if you already have 3 hours of rushes sync on a timeline and you don’t want to have to throw that work away, this is probably a workable bandaid.
The bug occurs when you subclip something originating from dual audio/video sources. So instead of making subclips, make a MERGED clip.
1. On your sunc timeline, trim in and out points of audio and video so that they are the same duration.
2. Grab both clips from the timeline and drag and drop them into the browser. They will appear as new, seperate audio and video file.
3. Highlight them both (they should already be highlighted), right click and select ‘Merge clips’ from the pop-up menu
4. Best to delete the 2 audio and video files you used to make this clip, to keep things tidy.
If anyone has a more elegant fix, please post! But in future, I would recommend using Avid for projects involving dual audio and video originating on different sources.