This is happening for me as well, it appears to be when using the Gutenberg plugin as opposed to core Gutenberg. Core does not pass a 3rd variable so $is_preview would expect to default to false in the ACF render callback, but the Gutenberg plugin passes the full block array as a parameter which ACF’s function currently does not expect. I’m currently checking true === $is_preview to determine if it’s a preview because ACF explicitly sets it to true when this is the case.
