Annotating Pattern Synonym Breaks Typechecking

Consider this module:

{-# LANGUAGE TypeFamilies
           , TypeFamilyDependencies
           , DataKinds
           , TypeOperators
           , GADTs
           , FlexibleContexts
           , PatternSynonyms
           #-}

module Strange where

data Expr a where
    Tuple :: NTup ts -> Expr (Flatten ts)

data NTup (ts :: [*]) where
    Unit :: NTup '[]
    Pair :: Expr a -> NTup ts -> NTup (a ': ts)

type family Flatten ts = r | r -> ts where
    Flatten '[] = ()
    Flatten '[a, b] = (a, b)

pattern P a b = Pair a (Pair b Unit)

fstExpr :: Expr (a, b) -> (Expr a, Expr b)
fstExpr (Tuple (P x y)) = (x, y)

GHC 8.6.4 accepts this program and infers the type Expr a -> Expr a1 -> NTup '[a, a1] for P. Annotating P with this type like so:

pattern P :: Expr a -> Expr b -> NTup '[a, b]
pattern P a b = Pair a (Pair b Unit)

Causes the program to be rejected with:

Strange.hs:26:17: error:
    • Could not deduce: ts ~ '[a, b]
      from the context: (a, b) ~ Flatten ts
        bound by a pattern with constructor:
                   Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten ts),
                 in an equation for ‘fstExpr’
        at Strange.hs:26:10-22
      ‘ts’ is a rigid type variable bound by
        a pattern with constructor:
          Tuple :: forall (ts :: [*]). NTup ts -> Expr (Flatten ts),
        in an equation for ‘fstExpr’
        at Strange.hs:26:10-22
      Expected type: NTup ts
        Actual type: NTup '[a, b]
    • In the pattern: P x y
      In the pattern: Tuple (P x y)
      In an equation for ‘fstExpr’: fstExpr (Tuple (P x y)) = (x, y)
    • Relevant bindings include
        fstExpr :: Expr (a, b) -> (Expr a, Expr b)
          (bound at Strange.hs:26:1)
   |
26 | fstExpr (Tuple (P x y)) = (x, y)
   |                 ^^^^^