Note the broken fontification for \next:nn and \cs_prefix_spec:N,
\cs_to_str:N.  This is mostly because : and _ have the symbol syntax in
this case and font-latex looks for word boundaries (\\>).  I think we
can cure this with this small change:

--8<---------------cut here---------------start------------->8---
diff --git a/style/expl3.el b/style/expl3.el
index 9ce0e4b0..f35088df 100644
--- a/style/expl3.el
+++ b/style/expl3.el
@@ -38,9 +38,9 @@

 (defvar LaTeX-expl3-syntax-table
   (let ((st (copy-syntax-table LaTeX-mode-syntax-table)))
-    ;; Make _ and : symbol chars
-    (modify-syntax-entry ?\_ "_" st)
-    (modify-syntax-entry ?\: "_" st)
+    ;; Make _ and : word chars
+    (modify-syntax-entry ?\_ "w" st)
+    (modify-syntax-entry ?\: "w" st)
     st))

 (defun LaTeX-expl3--set-TeX-exit-mark (_optional &optional pos)
@@ -710,7 +710,10 @@ Pass OPTIONAL, PROMPT and NO-PARAM to `TeX-arg-expl3-macro', which see."

                                 ("cs_undefine:N" "\\")
                                 ("cs_undefine:c" "\\"))
-                              'function)))
+                              'function))
+   ;; Also tell font-lock to update its internals
+   (setq font-lock-major-mode nil)
+   (font-lock-set-defaults))
  TeX-dialect)

 (defvar LaTeX-expl3-package-options-list
--8<---------------cut here---------------end--------------->8---

which then gives: