If you have disabled BBcodes in the ACP, quoting a message from viewtopic.php will switch to the post editor with that code in the textarea:

Big Monstro wrote:

> Content of the quoted message

However, if you quote a pm from ucp.php (not posting.php), while reading a private message (ucp_pm_viewmessage.html), you get that code in the post editor textarea:

[quote="Big Monstro" time=1523304530 user_id=2]Content of the quoted message[/quote]

That means the private message system doesn't take into account the nonavailability of BBCodes.