TypeApplications bug?

oh_no.hs

{-# LANGUAGE AllowAmbiguousTypes #-}
{-# LANGUAGE InstanceSigs        #-}
{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications    #-}
{-# LANGUAGE TypeFamilies        #-}

module Foo where

import           Control.Monad.Reader
import           Data.Kind            (Type)

-- non-injective type family
class Foo a where
  type Bar a :: Type

-- ambiguous class function
class Monad m => MonadFoo m where
  doTheThing :: forall a. Bar a -> m (Bar a)

-- use of type application in function to disambiguate: ok
liftFoo :: forall a m t. (MonadFoo m, MonadTrans t) => Bar a -> t m (Bar a)
liftFoo v = lift $ doTheThing @m @a v

-- use of type application in instance to disambiguate: KO!
instance MonadFoo m => MonadFoo (ReaderT r m) where
  doTheThing :: forall a. Bar a -> ReaderT r m (Bar a)
  doTheThing = liftFoo @a