iFrame sometimes needs to be called twice

Consider the following proof (written in the context of tests/proofmode_monpred.v):

  Program Definition monPred_id (R : monPred) : monPred :=
    MonPred (λ V, R V) _.
  Next Obligation. intros ? ???. eauto. Qed.

  Lemma test_iFrame_monPred_id (P Q R : monPred) i :
    (P i) ∗ (Q i) ∗ (R i) -∗ (P ∗ Q ∗ monPred_id R) i.
  Proof.
    iIntros "(? & ? & ?)". iFrame. iFrame.
  Qed.

Notice that I need to call iFrame twice. That should not be necessary.

This lead to a bunch of additional rewrites being introduced when fixing the fallout from https://gitlab.mpi-sws.org/FP/iris-coq/merge_requests/154 because the [$] specialization pattern stopped working (it does iFrame just once, it seems).