Proof mode notation broken depending on import order

It is a long-standing issue that sometimes, depending on the order of re-exports and imports, the proof mode notation is broken. I finally decided to minimize this, and you can find the result in the ipm-notation-broken branch. I first thought this is a Coq bug, but now I am not so sure anymore: We do have different notations in the same scope that lead to printing being ambiguous.

Namely, we have

Notation "P ⊢ Q" := (uPred_entails P%I Q%I)
  (at level 99, Q at level 200, right associativity) : C_scope.

and

Notation "Γ '--------------------------------------' □ Δ '--------------------------------------' ∗ Q" :=
  (of_envs (Envs Γ Δ) ⊢ Q%I)
  (at level 1, Q at level 200, left associativity,
  format "Γ '--------------------------------------' □ '//' Δ '--------------------------------------' ∗ '//' Q '//'", only printing) :
  C_scope.

We seem to be relying on a guarantee that if one notation is strictly more specific than another one, it will have higher priority. Does Coq claim to have such a guarantee?

Maybe one possible solution would be to move the proof mode notations into a different scope, and have iStartProof introduce that scope (i.e., turn the goal into (...)%ProofMode).