1. emacs -q
2. M-: (setq find-file-visit-truename t) RET
3. C-x C-f /sudo::/ RET
4. M-: default-directory RET

Expected value: /sudo::/
Actual: /sudo:://

The actual usability problem that this causes is that running find-file
again in the Dired buffer starts with a bad default:

Typing C-x C-f tmp RET results in finding /sudo:://tmp which is /tmp
rather than /sudo::/tmp

The immediate offending code is these lines in find-file-noselect

(if find-file-visit-truename
    (abbreviate-file-name (file-truename filename))
  filename)))

Tracing the bug:

(file-truename "/sudo::/")
(tramp-file-name-handler 'file-truename "/sudo::/")
(tramp-sh-file-name-handler 'file-truename "/sudo::/")
(tramp-sh-handle-file-truename "/sudo::/")

tramp-sh-handle-file-truename is (format "%s%s" A B), where A does most
of the work and B just adds an extra final slash conditionally:

   (if (string-equal (file-name-nondirectory filename) "") "/" "")))

I think this could be fixed by making it:

(let ((result A))
  (format "%s%s" result
                 (if (and (string-equal (file-name-nondirectory filename) "")
                          (not (string-equal (file-name-nondirectory
result) ""))) "/" "")))

I have attached a patch implementing this, made against emacs-26 since
this is a straightforward bug fix.

In GNU Emacs 26.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version
3.22.24), modified by Debian
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000
System Description: Debian GNU/Linux rodete (upgraded from: Ubuntu 14.04 LTS)

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES THREADS JSON LCMS2

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Dired by name