Kindle makes frame aroung page

Dear professionals,

help please to solve the problem.

I've made FXL mobi file and it is displayed perfectly in Kindle Previewer 3 on my computer. But when checking it in KDP Online Previewer, SOME pages have a thin border over margins. All images have the same border too.

When launching preview I got an alert:

This book contains interactive features that cannot be previewed with KDP Online Previewer. You can use Kindle Previewer 3 to preview this book with its interactive features, or continue previewing without interactive features.

But I don't have any interactive elements in Indesign document.

What should I do to remove the borders?

Thanks a lot.