Save something to global var in add_filter

I ran a test and the following works:

global $testMe;
$testMe = 0;
function my_acf_update_value( $value, $post_id, $field ) {
    global $testMe;
    $testMe = $value;
    return $value;
}
add_filter('acf/update_value/key=field_5308e5e79784b', 'my_acf_update_value', 10, 3);

// Test: Apply the filter
apply_filters('acf/update_value/key=field_5308e5e79784b','a','b','c');
// end Test

echo $testMe; // prints 'a'

So, in principle, your code is functional. There are a couple of things that I can think that may be going wrong.

1. You are adding the filter after the hook has ran. Place that test
   apply_filters before the rest of the code and it won’t work.
2. You have the hook name wrong.

Other than that, it should work. However, this global variable technique is a bit messy. Might I suggest something like this:

function my_acf_grab_value($value,$echo = false) {
  static $var = 0;
  if (false === $echo) {
    $var = $value;
  } else {
    return $var;
  }
}

function my_acf_update_value( $value, $post_id, $field ) {
  my_acf_grab_value($value);
  return $value;
}
add_filter('acf/update_value/key=field_5308e5e79784b', 'my_acf_update_value', 10, 3);

// Test: Apply the filter
apply_filters('acf/update_value/key=field_5308e5e79784b','Yay','b','c');
// end test

echo my_acf_grab_value('',true); 

Or this:

function my_acf_update_value( $value="", $post_id = 0, $field = '' ) {
  static $var = 0;
  if (!empty($value)) {
    $var = $value;
  } else {
    return $var;
  }
  return $value;
}
add_filter('acf/update_value/key=field_5308e5e79784b', 'my_acf_update_value', 10, 3);

// Test: Apply the filter
apply_filters('acf/update_value/key=field_5308e5e79784b','Yay','b','c');
// end test

echo my_acf_update_value();