Problem with importing video

I am trying to use Adobe Animate for a rotoscoping project and the reference video that I am trying to import I would like to keep at 15 frames per second. The problem is, after importing the video, (I set the document to 15 fps beforehand), it works fine but as soon as I save and close the project, the next time I open it the second half of my reference video is cut off, but still the same number of frames, and the video plays slower than before. Any help is appreciated!