get_terms returns incorrect terms set for indirect children

[mh35]

I created categories as below.

未分類(term_id=1) has two children, C1(term_id=2) and C2(term_id=3).
C1 has three children, C3(term_id=4), C4(term_id=5) and C5(term_id=6).
C2 has two children, C6(term_id=7) and C7(term_id=8).

If I query as below, it returns correct answer.

<?php
get_terms(array('taxonomy' => 'category', 'hide_empty' => false, 'name' => 'C1', 'child_of' => 1));

This query returns C1 term.

But I query as below, it returns no terms. I think this behavior is incorrect.

<?php
get_terms(array('taxonomy' => 'category', 'hide_empty' => false, 'name' => 'C4', 'child_of' => 1));

[mh35]

Bug cause situation

[joyously]

As it states in the parameter definitions on ​https://developer.wordpress.org/reference/classes/wp_term_query/__construct/

'child_of'
(int) Term ID to retrieve child terms of. If multiple taxonomies are passed, $child_of is ignored. Default 0.

So it looks like the correct result to me.

[mh35]

If I pass only taxonomy and child_of parameter, this function returns direct and indirect children.

<?php
get_terms(array('taxonomy' => 'category', 'hide_empty' => false, 'child_of' => 1));

This returns C1, C3, C4, C5, C2, C6 and C7. If I want to get only direct children, I pass parent parameter.

<?php
get_terms(array('taxonomy' => 'category', 'hide_empty' => false, 'parent' => 1));

This returns C1 and C2.