You can always use the Editor to fine-tune the book however you like. It is trivial to add an extra pagebreak or insert a blank HTML file containing a non-breaking space (assuming you know basic HTML).
(I will note that according to my personal preferences, blank pages are a bug and should be fixed. But to each his own.
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Without being able to see the underlying markup of the book in question, I cannot be sure why that would happen. (Please do not upload it here to show me if the book is copyrighted, that would be illegal. @jackie_w's
ScrambleEbook utility for troubleshooting ebooks was designed to fulfill that need.)
But I would guess the existing, broken coverpage has padding of some sort and possibly lines with a non-breaking space, which pad out the coverpage and overflow past what can appear on a single page.
When Polish Book replaced that page with its own coverpage template, the new coverpage did not have that artifact.
It was probably not deliberately done that way. But as I said above, if you for some reason prefer those aesthetics you can trivially fix it with the E-Book Editor.