The question is "why" is woocommerce loading that file in the checkout in the first place. Because in our templating we don't have any qty buttons on the checkout page. So basically I think both should be cleaned up (prevent loading, find out what actually loads the file in the first place) and the sanitation added as well.
I have to double check the 0 stuff. But otherwise code looks fine to me. Will try to do that asap

@Axos11 I'm new at woocommerce and I was just trying to build some custom discount functions with ACF while running in those warnings after pressing the checkout-button and it leads me into this file. So I was only trying to reach the checkout along this way. But yeah, the reason why should be good to know.

@neralex every contribution is welcome and thanks for bringing the focus back to that issue. I think that I would not change any of the values without need but just escape early if the data needed for the logic isn't present.

@crftwrk what do you think?

  add_filter('woocommerce_quantity_input_args', function (array $args) {

    add_filter('woocommerce_quantity_input_type', 'change_quantity_input_type');

    // Exit early if required args are not set
    if (!isset($args['max_value']) || !isset($args['min_value']) || !isset($args['input_value'])) {
      return $args;
    }
    // Rest of the code

@Axos11 I think it's great because 1. it fixes the bug and 2. it changes the actual behaviour less invasively than my patch does. #shortandsimple

I‘m in 👍 @neralex do you want to push a commit to this PR?

@crftwrk thanks for the offer but just merge it. @Axos11 found a great solution.

Got it!