The reason is that you cannot take a field that is not a sub field and then make it a sub field. This causes the meta_key of the field to change and disconnects any existing data from the field. You cannot alter the hierarchy of fields unless you change all the meta_key values for the fields and the acf field key references in the database.
This is not a bug, it’s how ACF works. You will find numerous topics here on the forum about how changing field names and recording fields in this way.
