If there is an empty string then usually that means that the post was saved with the acf field but no value was entered. So if the field name is correct then there must be some other reason why it’s not returning a value.
Where is the field group located? post, options page, somewhere else?
Where is the code for getting the field? Inside “The Loop” or outside?
Have you tried disabling other plugins or switching themes? There has been a rash of topics started lately about the divi theme and it’s data/time script breaking ACF.
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