URL parameter filtering with 'tag' parameter

[harolair]

Specifically why is the tag url filtering locked behind this check for $this->query_vars_changed. Seems like this could be removed from the condition to me.
This means post archive filtering using the tag url parameter doesn't work by default.
But if for example any query_var is changed in the 'pre_get_posts' filter it suddenly does work. Even if the query_var changed is entirely unrelated.

wp-includes/class-wp-query.php

class WP_Query {
  ...
  public function parse_tax_query( &$q ) {
    ...
    if ( '' !== $q['tag'] && ! $this->is_singular && $this->query_vars_changed ) {
       ...
    }
    ...
  }
  ...
}