Possibly inconsistent keys between mu/keys and mu/assoc

[vise890]

Hi there, first of all thanks for the fantastic library!

We've noticed a possible inconsistency between the values returned by malli.util/keys and the ones taken by malli.util/assoc:

(def S [:map [[:foo] :any]])

(ma/validate S {[:foo] 1})
;; => true

(mu/keys S)
;; => ([:foo])

(mu/assoc S [:foo] :int)
;; => [:map [[:foo] :any] [:foo :int]]

;; I'd have expected the above to return:
;; => [:map [[:foo] :int]]

;; Looking at the implementation of malli.core/-set-entries, I see that I can force it with:
(mu/assoc S [[:foo]] :int)
;; => [:map [[:foo] :int]]

A similar behaviour can be observed with mu/assoc-in and :multi with dispatch values wrapped in vectors.

This is not super intuitive. Is this intended behaviour?