It is said that Ymir spent about 60 years in Pure Titan form. It is also said that Pure Titan only have original subject's spine instead of full body like one of Nine (see ref  here or search word "spine" if number shifts).
So, how exactly she could recover back to complete human with brain, conciousness and memories while turning into one of Nine? Other transformations explicitly shown or mentioned so far generally only take less than hour between turning human into Pure and upgrading to Nine, so it less of a concern, but 60 years? Shouldn't pretty much of whatever was original Ymir be long lost by then?