The only reason I would think that it would stop working after the shapechange, is that previously it was stated in various places that spells cast on weapond with multiple striking surfaces, like spears with both sharpened ends, or animals with multiple attack forms (or humans), had to be placed on a specific end/attack. In the case above, the Ironhand would have most likely been cast to affect a punch or a kick, or a head butt and bears do not have those forms of attacks. If the spell was lets sy cast to improve the humans bite attack, and in certain situations that would make sense, then I do not see a reason as to why it not continue to work in bear form on the bite.