Unexpected compile error on type level lists with a single element

Tried this with ghc 8.0.2, 8.2.2 and 8.4.3 and all give an almost identical error on the following piece of code:

{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE DataKinds #-}
{-# LANGUAGE TypeOperators #-}
{-# LANGUAGE PolyKinds #-}
{-# LANGUAGE CPP #-}

{-# OPTIONS_GHC -Wall #-}

import Data.Singletons.Prelude.List

#if MIN_VERSION_singletons(2,4,0)
type (a :++ b) = a ++ b
#endif

data MyType = A | B | C | D | E | F

type TypeList1 = ['A, 'B]

type TypeList2 = TypeList1 :++ ['C, 'D]

-- Everything above is fine, but this line:
type TypeList3 = TypeList2 :++ ['F]
-- Gives the error:
--
--    type-level-list.hs:21:32: error:
--        • Expected kind ‘[MyType]’, but ‘[ 'F]’ has kind ‘*’
--        • In the second argument of ‘(:++)’, namely ‘[ 'F]’
--          In the type ‘TypeList2 :++ [ 'F]’
--          In the type declaration for ‘TypeList3’
--       |
--    22 | type TypeList3 = TypeList2 :++ ['F]

-- If instead I write it like:
type TypeList4 = TypeList2 :++ '[F]
-- I get the warning:
--
--    type-level-list.hs:33:34: warning: [-Wunticked-promoted-constructors]
--        Unticked promoted constructor: ‘F’.
--        Use ‘'F’ instead of ‘F’.

-- The following actually seems to work, but I don't understand why type level 
-- lists containing only one element are different.
type TypeList5 = TypeList2 :++ '[ 'F]

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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