Lambdas don't like quantified variables in body (other than their own)

[nunoplopes]

(assert (forall ((quant (_ BitVec 8)))
  (=
   (select (lambda ((idx (_ BitVec 8))) quant) #x01)
   #x00
  )
))
(check-sat)
(get-info :reason-unknown)

prints:

unknown
(:reason-unknown "Formulas should not contain unbound variables")

This is because it seems lambdas don't allow quantified variables in their body. Is there a fundamental reason why this is the case?

(I have a bunch of benchmarks failing on my side because I'm using lambdas to encode memcpy and memory may have quantified variables due to non-determinism)