Thumbnails of videos in iPad missing

I have a bunch of videos in the Photo app on my iMac. It's a 2019 iMac running the latest macOS.

For each video, I set a Poster Frame so that on the iMac I see the frame of choice as the thumbnail. This works fine.

However, if I sync with my iPad Pro, which runs the latest iOS, these Thumbnails are not synced. What I see as Thumbnail on the iPad is the first frame, which in my case is black.

I googled around to see if somebody posted a procedure on how to do this correctly. The only suggestion I found is to set a Key Photo for each video. I did that, but that also did not solve the problem.

Bottom line. Neither Making a Poster Frame nor Making a Key Photo nor both together enable the transfer of video Thumbnails from the iMac Photo app to the iPad Photo app.

Please somebody help. How do I get the Thumbnails of videos synced?