Google Play Books: Why are my ePub 3 covers not showing?

[CrankyLamb]

I recently converted some of my EPUB 2 e-books into EPUB 3 in Calibre, and adjusted the cover so that it is centered, thanks to the help of this community.

However, I have noticed that when I import some of these e-books to Google Play Books on my Android phone, the cover doesn't appear in the thumbnail.

To be clear, if I open the book, I will see the cover, but it doesn't appear in the thumbnail when I look at my bookshelf, as you can see in the screenshot below.

None of the other ePub Readers I've tried on both Android and iOS have this issue. When I import those same EPUB 3 converted e-books into my iPad, the cover appears correctly in the thumbnail on Apple Books.

How do I fix it?

I can send a copy of my e-book if you want to test it on your devices.