Personally I would discount sync issues. Having had a lot of them with large, slow motors they always presented themselves with rapid changes like stabbing the throttle or flying in blustery conditions. Your heli was doing a nice slow decent when it happened. Conditions look calm too.
I would change all cables on that arm. I’ve had a servo wire which had a bad crimp. A few crashes later I found it when I pulled on the wire and it came apart.