Irregular Aeson.TH results with ObjectWithSingleField

This might be a documentation issue. The documentation for ObjectWithSingleField says:

A constructor will be encoded to an object with a single field named after the constructor tag (modified by the constructorTagModifier) which maps to the encoded contents of the constructor.

Here's my test program:

{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE CPP #-}
module Main where

import Language.Haskell.Exts.Annotated
import Data.Aeson
import Data.Aeson.TH

$(deriveToJSON defaultOptions { sumEncoding = ObjectWithSingleField } ''ExportSpecList)
$(deriveToJSON defaultOptions { sumEncoding = ObjectWithSingleField } ''SrcSpanInfo)
$(deriveToJSON defaultOptions { sumEncoding = ObjectWithSingleField } ''SrcSpan)

Running ghc -ddump-splices --make Main.hs gives (1/3):

Main.hs:1:1: Splicing declarations
    deriveToJSON
      (defaultOptions {sumEncoding = ObjectWithSingleField}) ''SrcSpan
  ======>
    Main.hs:25:3-79
    instance ToJSON SrcSpan where
      toJSON
        = \ value_a81J
            -> case value_a81J of {
                 SrcSpan arg1_a81K arg2_a81L arg3_a81M arg4_a81N arg5_a81O
                   -> object
                        [((Data.Text.pack "srcSpanFilename") .= arg1_a81K),
                         ((Data.Text.pack "srcSpanStartLine") .= arg2_a81L),
                         ((Data.Text.pack "srcSpanStartColumn") .= arg3_a81M),
                         ((Data.Text.pack "srcSpanEndLine") .= arg4_a81N),
                         ((Data.Text.pack "srcSpanEndColumn") .= arg5_a81O)] }

An object, and a set of values that are of primitive types so no need to toJSON them. Ok, SrcSpanInfo next:

Main.hs:1:1: Splicing declarations
    deriveToJSON
      (defaultOptions {sumEncoding = ObjectWithSingleField})
      ''SrcSpanInfo
  ======>
    Main.hs:24:3-83
    instance ToJSON SrcSpanInfo where
      toJSON
        = \ value_a80U
            -> case value_a80U of {
                 SrcSpanInfo arg1_a80V arg2_a80W
                   -> object
                        [((Data.Text.pack "srcInfoSpan") .= arg1_a80V),
                         ((Data.Text.pack "srcInfoPoints") .= arg2_a80W)] }

An object, and a set of values that are not toJSON'd. The values are of type SrcSpan and [SrcSpan]. Ok, ExportSpecList next:

Main.hs:1:1: Splicing declarations
    deriveToJSON
      (defaultOptions {sumEncoding = ObjectWithSingleField})
      ''ExportSpecList
  ======>
    Main.hs:12:3-86
    instance ToJSON l_a6O3 => ToJSON (ExportSpecList l_a6O3) where
      toJSON
        = \ value_a7Us
            -> case value_a7Us of {
                 ExportSpecList arg1_a7Ut arg2_a7Uu
                   -> Array
                       (Data.Vector.create
                           (do { mv_a7Uv <- Data.Vector.Mutable.unsafeNew 2;
                                 Data.Vector.Mutable.unsafeWrite mv_a7Uv 0 (toJSON arg1_a7Ut);
                                 Data.Vector.Mutable.unsafeWrite mv_a7Uv 1 (toJSON arg2_a7Uu);
                                 return mv_a7Uv })) }

An array and a set of values that are toJSON'd.

Here are the datatypes cut and pasted from the Source-link on Hackage:

-- | A portion of the source, spanning one or more lines and zero or more columns.
data SrcSpan = SrcSpan
    { srcSpanFilename    :: String
    , srcSpanStartLine   :: Int
    , srcSpanStartColumn :: Int
    , srcSpanEndLine     :: Int
    , srcSpanEndColumn   :: Int
    }
  deriving (Eq,Ord,Show,Typeable,Data)

data SrcSpanInfo = SrcSpanInfo
    { srcInfoSpan    :: SrcSpan
--    , explLayout     :: Bool
    , srcInfoPoints  :: [SrcSpan]    -- Marks the location of specific entities inside the span
    }
  deriving (Eq,Ord,Show,Typeable,Data)

-- | An explicit export specification.
data ExportSpecList l
    = ExportSpecList l [ExportSpec l]
  deriving (Eq,Ord,Show,Typeable,Data,Foldable,Traversable,Functor,Generic)

I might be doing unsupported things, but in that case it would be nice to get a warning that I can't expect to deserialize my values in a regular way on the receiving end.

I'm using ghc 7.6.3, haskell-src-exts 1.15.0.1, and aeson 0.7.0.6 if that makes any difference.