Backpack error message prints out data from interface file instead of source file

unit sig where
  signature Sig where
    data A
    instance Show A

unit impl where
  module Impl where
    type A = Int -> Int

unit main where
  dependency sig[Sig=impl:Impl]
  module Main where
    import Impl
    main = print "no"

ghc --backpack Error3.bkp                                       
[1 of 3] Processing sig                                                                                                   
  [1 of 1] Compiling Sig[sig]         ( sig\Sig.hsig, nothing ) [Flags changed]                                           
[2 of 3] Processing impl                                                                                                  
  Instantiating impl                                                                                                      
  [1 of 1] Compiling Impl             ( impl\Impl.hs, impl\Impl.o ) [Flags changed]                                       
[3 of 3] Processing main                                                                                                  
  Instantiating main                                                                                                      
  [1 of 1] Including sig[Sig=impl:Impl]                                                                                   
    Instantiating sig[Sig=impl:Impl]                                                                                      
    [1 of 1] Compiling Sig[sig]         ( sig\Sig.hsig, sig\sig-Absk5cIXTXe6UYhGMYGber\Sig.o )                            
                                                                                                                          
sig\sig-Absk5cIXTXe6UYhGMYGber\..\Sig.hi:1:1: error:                                                                      
    * No instance for (GHC.Show.Show A)                                                                                   
        arising when attempting to show that                                                                              
        instance [safe] GHC.Show.Show A -- Defined at Error3.bkp:4:14                                                     
        is provided by `impl:Impl'                                                                                        
        (maybe you haven't applied a function to enough arguments?)                                                       
    * while checking that impl:Impl implements signature Sig in sig[Sig=impl:Impl]                                        
  |                                                                                                                       
1 | ☺??d♂90320210928?????☻???☺???☺?☺?☺☺☻☺☺☺☻☺☻??????☺????X?☺?Ã?????☺?????<???????☺???◄►???????????????☺?v????☺????????☺???
?????☺???????B???????☺☻???☻?☻♥?☻♦                                                                                         
  | ^                                                                                                                     

It seems the problem is that we decide to report information contained in the interface file Sig.hi, which doesn't make any sense.