Your checkbox data is saved as 1
or ''
if someone checked or unchecked it.
you can also verify this using var_dump($checkbox)
inside custom_checkbox_field_html
function
This should work.
function custom_checkbox_field_html(){
$checkbox = get_option( 'disabletitle_text' );
$is_checked = ( $checkbox != '' && $checkbox == 1 ) ? 'checked': '';
printf(
'<input type="checkbox" id="disabletitle_text" name="disabletitle_text" value="1" %s/>',
esc_attr( $is_checked )
);
}