I think this might be a bug with ACF Pro – I ran into the same issue. Relationship fields don’t appear to be saving correctly when the fields are attached to the User object.
An ACF relationship field on a Post saves as:
a:1:{i:0;s:3:”777″;}
While the same relationship field, made as a fresh field group, on a User saves as:
a:1:{i:0;s:3:777;}
I just spent 4 hours trying to figure out what was wrong with my meta query until I finally checked the database on a whim, and realized that the value isn’t saved the standard way to the User object (user_meta table).
Is there a reason why it saves in a different format to the User format than it does to the Post format?
