Out of curiosity…
function print_filters_for( $hook = '' ) {
global $wp_filter;
if( empty( $hook ) || !isset( $wp_filter[$hook] ) )
return;
print '<pre>';
print_r( $wp_filter[$hook] );
print '</pre>';
}
Called with:
print_filters_for( 'acf/update_value' );
Anything interesting in there? Is your function actually being added?
