I solved this by changing the priority when loading the admin_print_scripts function. Line 112 of image-watermark.php.
I went from
add_action( ‘admin_print_scripts’, array( $this, ‘admin_print_scripts’ ), 20 );
To
add_action( ‘admin_print_scripts’, array( $this, ‘admin_print_scripts’ ), 80 );
My assumption is WP6 changed the priority of jQuery loading which messed up the select list insertion.