This is a pretty simple and infuriating question. Can anyone help me understand why the tax_query fields below end up with different results? As far as I can tell, the only difference is the AND relation (which should not be necessary but doesn’t hurt) in the second tax_query. Anyone? I saw this post which may be having the same problem, but the answer didn’t do the trick.

(To be specific, these are the var_export results of query->get('tax_query') in pre_get_posts.)

No products returned (incorrect result):

array (
  0 => 
  array (
    'taxonomy' => 'product_cat',
    'terms' => 
    array (
      0 => 'Gift Cards',
    ),
    'field' => 'slug',
    'operator' => 'NOT IN',
  ),
  1 => 
  array (
    'taxonomy' => 'product_tag',
    'terms' => 
    array (
      0 => 'Construction:knit',
    ),
    'field' => 'slug',
  ),
)

Three products returned (the correct result):

array (
  0 => 
  array (
    'taxonomy' => 'product_cat',
    'terms' => 
    array (
      0 => 'Gift Cards',
    ),
    'field' => 'slug',
    'operator' => 'NOT IN',
  ),
  1 => 
  array (
    'relation' => 'AND',
    0 => 
    array (
      'taxonomy' => 'product_tag',
      'terms' => 
      array (
        0 => 'Construction:knit',
      ),
      'field' => 'slug',
    ),
  ),
)