That works only if the PC can reduce the NPC to 0 or negative APs in the one action. This suggests that, even if the NPCs had stood and fought, they would have been beaten that round anyway.
I think you are leaving the attempt to flee too late.
Remember, APs are not hit points. In RQ (IIRC), standard tactics was to flee only when low on hit points. In HW, being low in APs means you are almost beaten, regardless of what you want to do (fight OR flee). An NPC low on APs is exhausted or cornered --- they have few options.
Try choosing to flee when it has become clear that the NPCs are outclassed, but they still have a good stock of APs. Remember that they must WIN the extended contest to successfully flee. While trying to flee, they must decrease the PCs stock of APs to 0 while preventing their stock of APs being whittled to 0. If the NPCs have had no success against the PCs (that is, the PCs have lost few APs) and the sides were otherwise evenly matched (about the same initial APs) I think you would have to be make the decision when you had lost half your initial APs.
Clever mobile NPCs could choose to flee right from the start, wait for the PCs to waste APs trying to chase them, and THEN turn to fight. Hmm, I think my Vingan might try that one...
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