Thanks for this I just ran into the same problem! I am using the USaveGame object and the helpers in UGameplayStatics (like SaveGameToSlot), so I modified the code there to use that flag on the archive.
Epic - Whilst I understand the optimsation to not save out values the same as the defaults, I am a little confused as to why it doesnt delete the old value from the saved file. How does the old value that was different from the default, stay in the saved file, if the current property is now different (but identical to the default objects value).