If it's MOV being used for the movie, could that be the problem? Perhaps Apple MOV handling was changed in the Poser version upgrade from 2014 to 11? Can you apply some other movie clip format to a prop, and see if the problem remains?

My other thought is there may be some gamma shift going on. i.e. that the first frame starts by using the colour profile used for video-encoding, then Poser defaults to a system gamma / profile to play the rest of the movie?

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Learn the Secrets of Poser 11 and Line-art Filters.