opacity animation problems in Keynote 7.3.1

In moving from Keynote 6.6.2 to Keynote 7.3.1 (on High Sierra), I'm finding an issue with animating the opacity of movie files. Essentially, if you begin a movie with its opacity set at anything less than 100%, and then try to animate an opacity change to any other level, before the opacity animation takes place, the video instantly "pops" to 100% opacity and then executes the opacity animation. So, for example, if you're trying to slowly increase the opacity of a video from 0% upwards, you instead get a very jarring jump to 100% opacity.

(Interestingly, it seems that after a video has been displayed at 100% opacity, you can then animate its opacity down & back up smoothly.)

This worked fine for me in Keynote 6, but seems "broken" in Keynote 7. I've tried this on more than one computer and experienced the same problem. Are others seeing this? Is this a known problem? Any suggestions?