The PNP mirror sources approximately 1 mA into OUT. QN1 is biased by VIN DC so its emitter resistor demands a similar collector current; the output voltage settles where source and sink currents match while both transistors remain active.
Use .op first. If OUT is too near either rail, adjust the DC input by tens of millivolts - this illustrates why active-load stages are normally embedded in feedback or differential structures rather than left open-loop.
For AC, the collector sees ro of QN1, ro of QP2 and RL in parallel rather than a few-kilohm resistor. Estimate Av ~= -gm*(roN||roP||RL)/(1+gm*RE). Then verify transient headroom with the 5 mV input.
