Problems with a dynamic binding

[Alan Mackenzie]

Hello, Emacs.

I'm going crazy!

In emacs -Q (whether emacs-27 or master), put the following into
*scratch*:

    (let ((print-escape-control-characters t)) "\n\r\t")

, and type C-u C-x C-e.  The expected result would be:

    "\12\15\11"

, but instead this is painted onto the screen:

    "
    ^M      "

.  As far as I can tell, the dynamic binding of p-e-c-c is being
correctly made.  If I do

    (setq print-escape-control-characters t)

, followed by executing the `let' form, I get the expected string with
octal escapes.  It is as though the print routine is ignoring the
dynamic binding entirely.

Help!  Am I doing something blaringly obviously wrong, that everybody
but me can see right away?

Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).