You cannot sort of the title of the post object. The only thing that ACF stores about the post object field is the ID. The same principle applies for something like this as I posted about repeater sub fields here https://acfextras.com/dont-query-repeaters/
basically what you need to do is to create an acf/save_post filter, get the post object field and save the title of that post object as custom meta value associated with the post you want to be able to sort.
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