I'm not sure if this is an AC issue or a Unity issue but if a video is set to 'Prepare Only' before another video plays, there's a bug that occurs if the player minimises the game window and returns mid-play - it shows the first frame of the prepared video even while the original video is still playing.
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What are your versions for both AC and Unity, and on what platform? Are you attempting to prepare a video on the same VideoPlayer component as the one already playing?
Let's see screenshots of the two Engine: Play movie clip Actions involved.