Hi, there!

Looking at the potency of Amnesia from past content, the development team adjusted its duration for notorious monsters and content introduced after Seekers of Adoulin. Additionally, some of monsters before SoA were also adjusted.
Based on these changes they do not have any plans to make further adjustments to Amnesia at the moment, which includes making it possible to remove this status ailment this through Full Cure.

As mentioned previously in a different thread, it is not possible to add new spells currently so adding a new magic spell to remove Amnesia is not something we can address.