I may understand the reason.
Because User(User group) wasn’t given “unfiltered_html” capability.
https://codex.wordpress.org/Roles_and_Capabilities#unfiltered_html
I tried to check about other tags.
'span' => array(
'class' => array ()),
I allowed “span” tag and “class” attribute successfully.
'code' => array(
'class' => array ()),
“class” attribute was removed.
