In-scope assertion failure with unpacking + existential

Compiling ghc -O M causes an assertion failure in master (when assertions are enabled).

{-# LANGUAGE GADTs #-}
module A where

data Term where
  BinaryTerm :: (Show tag) => {-# UNPACK #-} !Bool -> !tag -> Term

f :: Term -> String
f (BinaryTerm _ tag) = show tag

  GHC version 9.9.20240217:
	ASSERT failed!
  in_scope InScope {}
  tenv [X0 :-> tag_aEr]
  tenvFVs {tag_aEr}
  cenv []
  cenvFVs {}
  tys [(# (# #) | (# #) #)]
  cos []
  Call stack:
      CallStack (from HasCallStack):
        assertPpr, called at compiler/GHC/Core/TyCo/Subst.hs:686:5 in ghc-9.9-inplace:GHC.Core.TyCo.Subst
        checkValidSubst, called at compiler/GHC/Core/TyCo/Subst.hs:718:29 in ghc-9.9-inplace:GHC.Core.TyCo.Subst
        substTy, called at compiler/GHC/Types/Id/Make.hs:1407:59 in ghc-9.9-inplace:GHC.Types.Id.Make
  CallStack (from HasCallStack):
    panic, called at compiler/GHC/Utils/Error.hs:507:29 in ghc-9.9-inplace:GHC.Utils.Error

Reproduced using commit a7569495. I found it when investigating #24463 (closed) but it should be independent.