So i've tried 3 other popular translation tools, and they have the exact same problem. There are now blog articles from these companies saying apples implementation of the .xliff standard is different in meaning to everyone else, and they've advised their users to not use these feature in iOS. To instead stick to using .strings files
Based on my limited understanding of this, I think I agree with them. Whatever I type into my storyboard is stuck there forever unless manually updated by hand, any attempt to update automatically is ignored? What about typos? or wanting to keep the IB image up to date with the latest english values to avoid developer confusion? I can see many cases where I would want the <source> to be updated. I definitely see it as a flaw/bug/oversight that there is no command line argument and/or button in the UI to say "yes I understand the risks, update all the source values to match the import". For it to simply be ignored and receive a silent warning in command line leaves no wiggly room. If tools manage the task of updating <source> across all the languages, I don't see why that should be a prevented workflow