Not familiar with this particular legion, IX Hispania. Sometimes a legion would get disbanded for fighting on the losing side of a Roman civil war, or backing the wrong horse in one of the later dynastic struggles. I wonder if a new emperor might just order the name of such a legion "stricken" from all record? According to the article there are a few tidbits surviving in the Netherlands, so maybe the censors missed a few?
The period of the Antonine Emperors was the most stable in the Empire. Essentially no civil conflict From Nerva until the death of Commodus. It was preceded by the Flavian dynasty from Vespatian to Domitian. Domitian was a tyrant who was assassinated and the Senate installed Nerva. Domitian succeeded the Julio-Claudian dynasty after the chaos following Nero. So all of the mess of civil war in Imperial times started in the 180’s, with the exception of 69 AD.