I’ve figured it out. It’s a bug with how ACF stores the “field_***” meta value for the relevant key. It appears you add the underscore to the option_name for the field reference and then apply a character limit after this to both the option_name for the value and for the field reference. E.g.
Obviously the _*** option_name needs one more char than the value option name. However, could this char limit lead to edge cases where two option names end up with the same name as only the truncated section of the option_name is different. Would it not be better to enforce a character count on the names themselves in the UI?
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