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 acf-field_64b38c4e3ec56-field_64c782fc4c55d
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.
