There is only one reason that I can thing of that the value is not saving, or maybe it is saving, but something is interfering with it. What is the name of the field? Is that field name duplicated anywhere?
If you have another field with the same name then when ACF attempts to get the field key of the field associated with the select field it could be getting the field key for the other field. This other field probably has a value that’s not listed as part of the select field so it is reverting to the default value.
