Internal error: ARR_WORDS object entered on ByteArray# & Unboxed Sum

Summary

The compiled program crashes at runtime. Either I am doing something terribly wrong with primops or there is a bug in GHC.

{-# LANGUAGE MagicHash                 #-}
{-# LANGUAGE UnboxedSums               #-}
{-# LANGUAGE UnboxedTuples             #-}
module Main (main) where

import GHC.Base

data MyArray t = MyArray (# t | ByteArray# #)

getBytes :: MyArray t -> ByteArray#
-- This would work on GHC versions < 9.0 when uncommented, but not on 9.0.1!
-- getBytes (MyArray (# | arr #)) = case runRW# (\s -> (# touch# arr s, arr #)) of (# _, r #) -> r
getBytes (MyArray (# | arr #)) = arr
getBytes _ = mkByteArray 13
-- Commenting out this NOINLINE pragma also makes it work successfully
{-#  NOINLINE getBytes #-}

mkByteArray :: Double -> ByteArray#
mkByteArray (D# x) = case runRW#
  ( \s0 -> case newByteArray# 8# s0 of
      (# s1, mba #) -> unsafeFreezeByteArray# mba ( writeDoubleArray# mba 0# x s1)
  ) of (# _, ba #) -> ba

main :: IO ()
main = print $ case getBytes x of a ->  D# (indexDoubleArray# a 0#)
  where
    x :: MyArray Double
    x = MyArray (# | mkByteArray 7 #)

Here is what I get:

$ cabal v2-run enter-words
Resolving dependencies...
Build profile: -w ghc-9.0.1 -O1
In order, the following will be built (use -v for more details):
 - enter-words-0.1.0.0 (exe:enter-words) (configuration changed)
Configuring executable 'enter-words' for enter-words-0.1.0.0..
Preprocessing executable 'enter-words' for enter-words-0.1.0.0..
Building executable 'enter-words' for enter-words-0.1.0.0..
enter-words: internal error: ARR_WORDS object (0x42001060b8) entered!
    (GHC version 9.0.1 for x86_64_unknown_linux)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
Aborted (core dumped)

Maybe it's just me using incorrectly runRW#/newByteArray#/unsafeFreezeByteArray# or unboxed sums? I am sorry that I spotted it in my project two years ago, but failed to write a small example program to report. Now this is more critical, because the workaround does not work anymore.

Steps to reproduce

Compile and run the program with GHC 8.4+

Expected behavior

The program is expected to output a single line "7.0" and finish successfully.

Environment

• GHC version used: 8.4.4, 8.6.5, 8.8.4, 8.10.4, 9.0.1

Optional:

• Operating System: Ubuntu 20.04.2 LTS
• System Architecture: x86_64