WordPress not opening posts with only numbers if permalink is post_name

Sorry I didn’t follow up on this. You mentioned you weren’t using date archives. You can change the date archive structure to remove the conflict with /%postname%/:

function wpa_change_date_structure(){
    global $wp_rewrite;
    $wp_rewrite->date_structure="date/%year%/%monthnum%/%day%";
}
add_action( 'init', 'wpa_change_date_structure' );

So now after permalinks are flushed, date archive URLs will be prefixed with date/, so numeric postnames will no longer match the date rewrite rule.