Steps to reproduce the issue

create array:

        $items = [];
        $items[] = ['column_a' => 1, 'column_b' => 2, 'column_c' => 3];
        $items[] = ['column_a' => 4, 'column_b' => null, 'column_c' => 6];

use ArrayHelper::dropColumn to drop column_b from the array

var_dump(ArrayHelper::dropColumn($items, 'column_b'));

Expected result

column column_b removed from array

array(2) {
  [0]=>
  array(2) {
    ["column_a"]=>
    int(1)
    ["column_c"]=>
    int(3)
  }
  [1]=>
  array(3) {
    ["column_a"]=>
    int(4)
    ["column_c"]=>
    int(6)
  }
}

Actual result

column column_b partially removed from array resulting in unexpected behavior / result

array(2) {
  [0]=>
  array(2) {
    ["column_a"]=>
    int(1)
    ["column_c"]=>
    int(3)
  }
  [1]=>
  array(3) {
    ["column_a"]=>
    int(4)
    ["column_b"]=>
    NULL
    ["column_c"]=>
    int(6)
  }
}

System information (as much as possible)

Joomla 4.2.6 but probably already from version 1.5 (as that is when this function was added)

Additional comments

Issue is caused both for arrays and object. The function uses isset() to determine if a key is set in the object / array and if set remove that. But isset() also checks the value, when that is null (like in the example given) then it will also return false > resulting in the key not being unset in the function.

for arrays array_key_exist() and for object property_exists() should be used to determine if key is set.