If you go to Settings and there to Preferred Applications you can have a look which reader you use for ePub. It is possible to choose from all 3. The default is probably Pdf Reader.
And place your bug in the section of reader that you have set in M92.
I tried Pdf Reader with pdf document and it behaves correctly. Well, the question is what is correctly.
It goes to the 1st page, if one does not fill the page field. I guess, better would be do nothing and stay at current page, but OK. Why not, like this.