A big sentiment that makes alot of merchants shy away from apps unfortunately not just customer account related ones. The way data privacy is shaping at some point their may need to be an affordance for these types of situations to obscure/restrict specific information served to apps.
So maybe, in any future api feedback about merging through apps , hashing(vaulting?) should be brought up as a first class feature.
Since in it’s crudest forms such apps would either be doing manual merge where nothing is even parsed because the merchant|staff is doing the confirmation, or trying to automatically merge by string matching some fields; so in the automatic case it wouldn’t matter if the info is hashed between dupe accounts if it’s the same string it will create the same hash. Fuzzy matching or inferencing would be a different issue.