It could be that the video is so compressed that it is “off”.
Add too that a not-compensated screen delay calibration, and you might see what you see. (Buy yourself the “Catchinsync” app)
Try converting the video to Dndx of ProRes, and see what gives. Together with Photo Jpeg, these are the only codecs that will guarantee you proper video sync.
And lastly, you can avoid many problems by making a document with your delivery specs.
That should (at least) include proper compressed video (DnDx / ProRes / Photo Jpeg), BITC and 2pop at frons and end.
That’ll allow you to accurately identify any problem.
And avoid so many other problems at a later stage. (Version numbers, miscommunication of Timecodes, etc …)
Just my 2 cents. (Know that we work with video almost 24/7)
Fredo