I recently loaded this file into GHCi:

{-# LANGUAGE GADTs #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE TypeFamilies #-}
module Bug where

import Data.Proxy

data Foo1 (a :: k) where
  MkFoo1a :: Proxy a -> Int -> Foo1 a
  MkFoo1b :: { a :: Proxy a, b :: Int } -> Foo1 a

data family Foo2 (a :: k)
data instance Foo2 (a :: k) where
  MkFoo2a :: Proxy a -> Int -> Foo2 a
  MkFoo2b :: { c :: Proxy a, d :: Int} -> Foo2 a

And when I queried these datatypes with :info, I saw this:

$ ~/Software/ghc/inplace/bin/ghc-stage2 --interactive Bug.hs -fprint-explicit-kinds
GHCi, version 8.7.20181129: http://www.haskell.org/ghc/  :? for help
Loaded GHCi configuration from /home/rgscott/.ghci
[1 of 1] Compiling Bug              ( Bug.hs, interpreted )
Ok, one module loaded.
λ> :i Foo1 Foo2
type role Foo1 nominal phantom
data Foo1 @k (a :: k) where
  MkFoo1a :: forall k (a :: k). (Proxy @{k} a) -> Int -> Foo1 k a
  MkFoo1b :: forall k (a :: k).
             {a :: Proxy @{k} a, b :: Int} -> Foo1 k a
        -- Defined at Bug.hs:8:1
data family Foo2 @k (a :: k)    -- Defined at Bug.hs:12:1
data instance forall k (a :: k). Foo2 @k a where
  MkFoo2a :: forall k (a :: k). (Proxy @{k} a) -> Int -> Foo2 @k a
  MkFoo2b :: forall k (a :: k).
             {c :: Proxy @{k} a, d :: Int} -> Foo2 @k a
        -- Defined at Bug.hs:13:15

Yuck. A couple of icky things to note here:

• For some reason, the Proxy @{k} a field is needlessly parenthesized in MkFoo1a and MkFoo2a. This does //not// happen when record syntax is used, as demonstrated with MkFoo1b and MkFoo2b.
• Even more strangely, despite the fact that k is a specified argument, we're pretty-printing the return type of MkFoo1{a,b} as Foo k a, not Foo @k a. This problem doesn't appear to happen for data family instances, since MkFoo2{a,b} don't have this issue.

Patch incoming.