Your posts are full of conflicting information.
You are trying to update a field with the key
field_64c782fc4c55d and that it is not a sub field.
Later you say that the ID of the field is
This clearly shows that this field is a sub field of some kind because that is the only reason that the ID would contain 2 field keys.
At this point I really have not idea what your field are or what you are working with, nor can I tell why the update is not working.
Please provide a PHP export of the field group containing this field.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users
Helping others is a great way to earn karma, gain badges and help ACF development!