(s/def :corp/employee
  (s/keys :req [:employee/first-name]))

(s/def :employee/first-name string?)

(s/def :favorite/ice-cream #{:chocolate :vanilla})

(e/expound :corp/employee {:employee/first-name "michael"
                           :favorite/ice-cream :strawberry})

;; -- Spec failed --------------------

;; {:employee/first-name ...,
;;  :favorite/ice-cream :strawberry}
;; ^^^^^^^^^^^

;; should be one of: :chocolate, :vanilla

;; -- Relevant specs -------

;; :favorite/ice-cream:
;; #{:chocolate :vanilla}
;; :corp/employee:
;; (clojure.spec.alpha/keys :req [:employee/first-name])

;; -------------------------
;; Detected 1 error

given that the :corp/employee spec doesn't say anything about ice cream

That is by design in Spec — and the reference docs on http://clojure.org talk about that. I think the rationale may do too.