While the Magnate is a Regent, Regents other than the Magnate cannot tax Empire-controlled cities (even Loyal ones) unless they have the matching Monopoly.
If the Magnate becomes an Outlaw, flip this to Open Economy.
Act: 3
A: No. A Regent Lord is bound by Truce law when taxing a Regent Vassal. An Outlaw Lord can always tax Vassal cities regardless of Empire presence.