Reply To: two issues more: multiselect filter & Tag filter

In that code you are not using tags, that is the “books” taxonomy:

'tax_query' => array(
    array(
      
      'taxonomy' => 'books',
      'field' => 'slug',
      'terms' => 'พระวินัยปิฎก'
)
 

That is a taxonomy filter on “books”, not the tag filter.