Answer
Check two things in the URL. First, query parameters need to be separated from the path using a question mark (?), not a semicolon (;) — a URL like page/SomePage/List;$filter=... needs to be page/SomePage/List?$filter=... instead, since the semicolon form doesn't get parsed as a proper query string. Second, make sure you're referencing the page parameter by its actual Name (the technical identifier), not its Description (the human-readable label shown in the UI) — using the Description in the URL is what causes the placeholder to be passed through literally instead of being substituted with the real value.