Odd interation between ambiguity check, instance constraint, and fundeps

Summary

I suspect this might be related to #21149 (closed), but it doesn't look quite the same so I figured I'd report it anyway. A custom type error is reported in a situation where it's not at all obvious why the TypeError in question is even being inspected.

Steps to reproduce

{-# language DataKinds, PolyKinds, FunctionalDependencies, AllowAmbiguousTypes, UndecidableInstances #-}
module AmbTE where
import GHC.TypeLits
import Data.Proxy

class xs ~ (hd ': tl) =>
      HeadTail (msg :: ErrorMessage) (xs :: [k]) (hd :: k) (tl :: [k])
     | xs -> hd tl, hd tl -> xs
instance xs ~ (hd ': tl) => HeadTail msg xs hd tl
instance {-# INCOHERENT #-}
    (TypeError msg, '[] ~ (hd ': tl)) => HeadTail msg '[] hd tl

foo :: forall {k} (xs :: [k]) (hd :: k) (tl :: [k]). HeadTail ('Text "whoopsy") xs hd tl => Proxy '(hd, tl)
foo = Proxy

This reports

AmbTE.hs:10:8: error:
    • whoopsy
    • In the ambiguity check for ‘foo’
      In the type signature:
        foo :: forall {k}
                      (xs :: [k])
                      (hd :: k)
                      (tl :: [k]). HeadTail ('Text "whoopsy") xs hd tl => Proxy '(hd, tl)
   |
10 | foo :: forall {k} (xs :: [k]) (hd :: k) (tl :: [k]). HeadTail ('Text "whoopsy") xs hd tl => Proxy '(hd, tl)

Since I don't see any '[] in that type signature, it seems rather surprising to get the type error!

After writing the above, I realized that the fundeps are actually redundant. Removing them, surprisingly, makes it compile!

Expected behavior

I'd expect the above to compile.

Environment

• GHC version used: 9.4.1

Optional:

• Operating System:
• System Architecture: