I have no actual experience with the microcurrent but nevertheless I'm willing to suggest that that this is normal operation. Just as any ordinary multimeter uses progressively larger value shunt resistors with a corresponding increase in burden voltage, the microcurrent must play by the same rules, it is merely designed mainly to minimize these losses where insertion of such a measurement device might upset normal operation of the circuit, mainly an issue with low voltage, relatively high current circuits, 3.3 volts, 2.5. 1.2, etc.