Existential scope escape using variable name

[Bowuigi]

{-# LANGUAGE BlockArguments #-}
module Main where

import Data.STRef
import Control.Monad.ST

data Ex where
  Ex :: STRef r Int -> Ex

-- Pack `STRef s Int` into an existential, hiding the value of `s` (and Int but that's irrelevant)
ok :: Ex
ok = runST do
  ref <- newSTRef 1
  return (Ex ref)

-- Precisely what shouldn't happen
-- Do note that if I rename `r` to `s` it signals a (correct) error
-- (∃r. STRef r Int) -> (∀r. STRef r Int)
wrong :: Ex -> forall r. STRef r Int
wrong (Ex ref) = ref

-- Same here: Renaming r to s signals a (correct) error
wrong2 :: Ex -> forall r. ST r Int
wrong2 (Ex ref) = readSTRef ref

main = putStrLn "Hello!"

Renaming r to s signals this error (correct behavior):

*** Exception: error: "./Main.hs": line 21, col 18: Cannot satisfy constraint: r ~ s
     fully qualified: Primitives.~ r s

GHC raises this error when loading the original version:

Main.hs:21:18: error: [GHC-25897]
    • Couldn't match type ‘r1’ with ‘r’
      Expected: STRef r Int
        Actual: STRef r1 Int
      ‘r1’ is a rigid type variable bound by
        a pattern with constructor: Ex :: forall r. STRef r Int -> Ex,
        in an equation for ‘wrong’
        at Main.hs:21:8-13
      ‘r’ is a rigid type variable bound by
        the type signature for:
          wrong :: Ex -> forall r. STRef r Int
        at Main.hs:21:1-20
    • In the expression: ref
      In an equation for ‘wrong’: wrong (Ex ref) = ref
    • Relevant bindings include
        ref :: STRef r1 Int (bound at Main.hs:21:11)
   |
21 | wrong (Ex ref) = ref
   |                  ^^^

(And then another one for wrong2 but it's basically the same error)