Hi @Vasili
Thanks for the info.
It is most likely that within a widget, ACF does not know which post to load the value from.
You can test this by adding the following code above global $post;
$post_id = get_field_ID();
echo '<pre>';
var_dump( $post_id );
echo '</pre>';
Can you check the variable value and compare against the expected post ID?
Thanks
E