ACF finding the data is dependent on a combination of the field name (meta_key) and the field_key for each field.
In addition to this there was a change not long ago that effectively makes in impossible to get field values for fields not defined by an acf field so getting a value for a field depends on field_key existing and corresponding to the field name.
If either the field key or field name is changed then it is very likely you will not be able to get the data saved under the old field name/field key combination.
