Version check on branch is actually broken. But it has not been noticed since we have not had any updates since it was implemented.

But the new get_update_on_branch() method actually returns a 1-dimensional array, not a multi-dimensional array as does get_suggested_updates(), so it is not being passed to the template correctly, resulting in

PHP Fatal error: Uncaught TypeError: Argument 2 passed to phpbb\template\base::assign_block_vars() must be of the type array, string given, called in /phpBB/includes/acp/acp_update.php on line 58 and defined in /phpBB/phpbb/template/base.php:110