Voltage dip on MOSFET switchable current shunt
Due to parasitic capacitance within the MOSFET (from gate to drain), when you drive the gate low (in order to turn it on), this transient is coupled through to the output and you see a short spike as per your simulation. I'd say this should be expected. If you want to get rid of (or reduce) this effect try slowing down the activation signal mAEn or uAEn.

Also, as a footnote, don't expect this type of circuit to work well when there is a high frequency AC superimposed on the DC because the MOSFET's parasitic drain-source capacitance will conduct some current when the MOSFET is supposedly inactive.