Well, it is a switching regulator, so... if you apply a large load to the output, then remove that load, it is likely to overshoot. With an undersized output filtering stage, it's entirely possible that its regulation suffers when faces with current transients. Now, if the voltage increases steadily with a consistent heavy load on the output (say, a 5 ohm resistor) then it's probably a component behaving out of tolerance. Maybe a saturated inductor? Or comparator oscillation? I don't know.
It would be interesting to put a scope on the feedback voltage divider to see what the comparator input looks like under overload conditions.
Whether this is "abnormal abuse" or something that should be handled properly by the IC's protection circuitry is debatable. I would like to know that if something in the load circuit failed, the PSU would do everything it its power to behave according to the three-tiered model: Protect the user, protect the load, and protect itself -- in that order. As a realist, I don't necessarily expect that in a regulator bought for spare change from a no-name overseas merchant via eBay.