Post by category+author not working!

Question

why this code is ignoring the author id

It’s not ignoring the author ID. But with that code, you need to call setup_postdata(), like this:

//global $post; // Uncomment if necessary.

$current_user = wp_get_current_user();
//$current_user->ID;

$post_ref = get_posts( array( 'author' => $current_user->ID, 'numberposts' => 1, 'cat' => 31 ) );
foreach( $post_ref as $post ):
  setup_postdata( $post ):
  the_title(); echo get_the_ID();
endforeach;
wp_reset_postdata();