I had the same problem. Do NOT use preview as affirmation of A/V Sync. Go by your wave forms and Markers. See my bug report also. In my case it was 6 frames off. I just did a video with my iphone 8 instead of my go-pro and once again preview audio was slightly out of sync, but my wave-forms line up. Exported with perfect audio sync in Pro-Res 4:2. Then I use handbrake to get that down to a more manageable size.