My proxy and original have the same total duration, but toggling proxies shifts picture against audio. The source MP4 is VFR. Does a proxy need to preserve more than just start/end length?
Yes. Matching total duration is not enough if timing differs throughout the clip. VFR is especially troublesome when a proxy is created as simplistic CFR without correct time mapping.
Compare a visible sync event near the start, middle and end with proxies toggled. Growing error means drift, not one fixed offset.
Fix source timing first or use a proxy workflow that explicitly supports the VFR source. Otherwise edits made against the proxy can point at different source moments.
Action Video Lab Editorial Team
The start matches but the end is far off. I will conform the VFR source correctly before generating new proxies.