Video Player Start Screen in Rise 360 is blank

Hello community,

I created a slide in PowerPoint which contains a short video of chefs in a kitchen (which I set to playback automatically in PPt when the slide starts.) Then I also inserted on the same slide two rectangles with text which have timed fade-in and grow/shink animations applied to them. I then took this single-slide PPt presentation and, after optimizing and compressing the media, exported it as a 720 (HD) video.
When I upload this mp4 (which contains the animation) as a video block in Rise 360, the initial screen which the viewer sees in the preview and in the published course is blank (solid black). The player controls are there and the video does play back when the viewer clicks on the play button; the only problem is the initial screen being black instead of showing the initial static image of the chefs standing in the kitchen.
Just to compare, after I experienced this problem, I created a separate presentation with the exact same video inserted on a PPt slide and then exported as a video the precise same way as the previous slide. The only difference is that this slide did not have any shapes or animations. When I insert this video in Rise, the initial screen does show the initial static screen (frame?) of the chefs instead of the blank screen. As with the other video, choosing the Play video button starts the video.
It seems like, as far as Rise is concerned, both of these assets should just be recognized as the same thing: an mp4 video. (When viewed in Windows 10 File Explorer, both files show the starting image of the chefs.) Any thoughts as to why one start screen is appearing as black while the other is showing the video’s starting image?

Thanks