To reproduce:

emacs -Q
In the scratch buffer type:

(condition-case nil
    (fail-badly)
  (error (when foo t)))

I don't see WHEN with font-lock-keyword-face.  However:

(condition-case nil
    (fail-badly)
  (error (if foo t)))

IF has font-lock-keyword-face.


In GNU Emacs 28.0.50 (build 181, x86_64-pc-linux-gnu, cairo version 1.15.10)
 of 2020-09-07 built on tbb-desktop
Repository revision: 0ebe2678002ffb82a25311c56cbc4b8ba3bd5fa1
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Ubuntu 18.04.5 LTS

Configured using:
 'configure --with-x-toolkit=no'

Configured features:
XPM JPEG TIFF GIF PNG CAIRO SOUND DBUS GSETTINGS GLIB NOTIFY INOTIFY
GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB OLDXMENU X11 XDBE XIM MODULES
THREADS PDUMPER

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

Major mode: Lisp Interaction

Minor modes in effect:
  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 mule-util info emacsbug message rmc puny dired
dired-loaddefs rfc822 mml mml-sec epa derived epg epg-config gnus-util
rmail rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search seq byte-opt
gv bytecomp byte-compile cconv mm-decode mm-bodies mm-encode mail-parse
rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045
ietf-drums mm-util mail-prsvr mail-utils time-date subr-x pcase misearch
multi-isearch vc-git diff-mode easy-mmode bug-reference pp wid-edit
descr-text help-fns radix-tree cl-print debug backtrace help-mode
easymenu find-func cl-loaddefs cl-lib tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode elisp-mode lisp-mode
prog-mode register page tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
term/tty-colors frame minibuffer 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 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 threads dbusbind inotify dynamic-setting system-font-setting
font-render-setting cairo x multi-tty make-network-process emacs)

Memory information:
((conses 16 133020 8093)
 (symbols 48 8406 1)
 (strings 32 28861 5704)
 (string-bytes 1 864362)
 (vectors 16 14411)
 (vector-slots 8 187982 17929)
 (floats 8 37 39)
 (intervals 56 17238 0)
 (buffers 992 17))