I fixed your plugin

Your function for updating the cart in version 1.3 of your plugin is broken.

The function is wcmmq_s_update_cart_validation in file includes/set_max_min_quantity.php.

The fixes are in lines 96 and 100 and the fixed code looks like this:

96: }elseif( $max_quantity && $quantity > $max_quantity ){

100: }elseif( $min_quantity && $quantity < $min_quantity ){

Without that fix, a user cannot change the cart quantity for a product that has no minimum and maximum set.