Subscription-Data is a mandatory AVP within the IDR command, however sub AVPs of this grouped AVP are optional. So HSS can add Subscription-Data AVP without having any sub AVPs while sending IDR to MME. Specification covers the handling of this scenario.
In this particular case, MME will continue with the stored subscription.
In essence, if Subscription-Data AVP includes any sub AVP then MME considers the new value of that parameter.