ACF has altered the way is stores and returns values. This was done when the new date/time and time fields were added in order to make dates compatible across these field types.
YYYYMMDD is the way that ACF used to store dates. YYYY-MM-DD is the way that is stores dates now. When loading the value for a field ACF checks to see if the format is the second one and if not it adds the dashes for compatibility, but it does not update the existing value unless you edit the old post where the date is stored and update it.
What you’ll need to do is check the formatting and add the dashes yourself if you are getting values without using ACF.
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