Hi Everyone,
I had same issue, like this:
https://forums.garmin.com/developer/connect-iq/f/discussion/5994/animated-gif-as-cover-in-the-app-store
But I have found a link, what can convert images to correct file.
It is:
https://ezgif.com/gif-to-apng
So, you should convert to APNG, NOT TO GIF!
This is works fine for me.
Bye