hmmmm, yes, I suspect the cited pages field messes with the grouping, as it needs to be applied to a specific citation and cited pages in numbered formats are unusual. What if you use “before” or “after” fields rather than the cited pages field? How do the citated pages appear when you do use them in the numbered format? Can you instead edit the settings for Author (Year) for the unusual ones where you need a cited page? That might look like (Ref Bibliography number, page Cited Pages)
(Ref #, page X)