Enable SQL debug explain and x-debug to catch it.

Or remove the error control operator "@" here https://github.com/phpbb/phpbb/blob/master/phpBB/phpbb/db/driver/mysqli.php#L327 and visit the FAQ page or any controller page.

The DB connction gets closed here in this line: https://github.com/phpbb/phpbb/blame/3.3.x/phpBB/includes/functions.php#L4496

Which seems can be removed, awaiting confirmation.