It is highly unusual to use cited pages with a superscripted bibliography number. Is that what you are trying to do?
Also I remember having issues with parentheses and cited pages in citations alot, particularly in numbered styles, but even in author year styles. (see Numbered references in alphabetical order thread for example). It probably has something to do with punctuation handling rules and developers not testing some scenarios. Also, that citations can be groups of citations, and not always stand alone. All in all, I think there is still an inherent glitch. I really think your “bypass” option is the best way forward. Since you have to manually enter cited pages anyway. As to “why” - it seems to work within parentheses when using the Author(Year) type of citations, but with numbers or leaving them alone in parentheses, there is some unsolvable glitch.