I don't have a standalone way to reproduce the problem but the following
is quite easy if you have the all-the-icons and memoize packages installed.
Start with 'emacs -Q' and from the *scratch* buffer.

(require 'memoize)
(require 'all-the-icons)

(all-the-icons-insert-alltheicon)

At this point, a list of icons is offered from the "all-the-icons.ttf"
 font. The glyph is correct. Select anyone of these glyphs and when it
 is inserted in the *scratch* buffer, the font face is lost and you get
 an empty glyph.

This used to work at some point in emacs 26.0.50.

Regards,

Fabrice


In GNU Emacs 26.0.91 (build 2, x86_64-w64-mingw32)
 of 2018-02-22 built on MARVIN
Repository revision: 9ff763484212d25cbdfc2f5921ba1e7bbdbdab10
Windowing system distributor 'Microsoft Corp.', version 10.0.17074
Recent messages:
Loading c:/Home/.emacs.d/elpa/memoize-20170720.1802/memoize.el
(source)...done
Load: 1 file.
Load * [2 files]? (y or n) y
Loading c:/Home/.emacs.d/elpa/all-the-icons-20180125.757/all-the-icons.el
(source)...done
Loading
c:/Home/.emacs.d/elpa/all-the-icons-20180125.757/all-the-icons-faces.el
(source)...done
Load: 2 files.
Mark set
Making completion list... [2 times]
Quit
Making completion list...

Configured using:
 'configure --prefix=/MingW64 --with-jpeg --with-xpm --with-png
 --with-tiff --with-rsvg --with-xml2 --with-gnutls --with-imagemagick
 --without-dbus --enable-checking=no 'CFLAGS=-I/mingw64/include -O3
 -ggdb3 -mtune=corei7' CPPFLAGS=-I/mingw64/include
 LDFLAGS=-L/mingw64/lib'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND NOTIFY ACL GNUTLS LIBXML2
ZLIB TOOLKIT_SCROLL_BARS THREADS LCMS2

Important settings:
  value of $LANG: fr_FR
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  diff-auto-refine-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt bytecomp
byte-compile cconv format-spec rfc822 mml mml-sec password-cache epa
derived epg epg-config gnus-util rmail rmail-loaddefs mm-decode
mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader
sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
all-the-icons data-material data-weathericons data-octicons
data-fileicons data-faicons data-alltheicons memoize cl-macs gv
all-the-icons-faces dired-aux cl-loaddefs cl-lib vc-git diff-mode
easymenu easy-mmode view dired dired-loaddefs elec-pair time-date
mule-util tooltip eldoc electric uniquify ediff-hook vc-hooks
lisp-float-type mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win
w32-vars term/common-win tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page menu-bar rfn-eshadow isearch timer select
scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite charscript charprop case-table epa-hook jka-cmpr-hook
help simple abbrev obarray minibuffer cl-preloaded nadvice loaddefs
button faces cus-face macroexp files text-properties overlay sha1 md5
base64 format env code-pages mule custom widget hashtable-print-readable
backquote w32notify w32 lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 121440 10571)
 (symbols 56 21522 1)
 (miscs 48 63 120)
 (strings 32 39934 1637)
 (string-bytes 1 894990)
 (vectors 16 15440)
 (vector-slots 8 515327 10180)
 (floats 8 316 308)
 (intervals 56 890 2202)
 (buffers 992 15))