I watched the following behavior often at least one year with Okular binaries from different distributions (Debian, openSUSE Tumbleweed, Antergos): Scrolling through the pages, via arrow key or via scrollbar, when the page number exceeds 9 the page number is not viewed properly anymore in the textfield, the textfield did not changed it size but the page number now need more place to be showed and so parts of the page number are not visible until window resize.
I remember seeing this as well, but it was fixed eventually (I don't remember when exactly). Can you try a newer version of Okular?
I tried version 1.0 and there the problem obviously does not exist anymore.
Thanks for the update; closing.