Interesting. That kind of suggests the mp4 start date/time header has a 2 second error when the Mapillary firmware is used, but ONLY when server side processed. I can see some code/processing workaround might work if the error was constant.
Which of your methods produces the correct result? Based on the map it seems that “locally” is the closest.
Are you using the standard 5 frames per second config? I am running 2.