Unable to upload existing translations to xtm

How to reproduce

1. Checkout https://gitlab.com/wspee/xtm-cms-dummy/tree/12-add-existing-translations
2. Note that the string string1 already has a translation:

$ cat locales/test.json 
{
  "string1": {
    "message": "string1-de"
  }
}

1. Create XTM project using: python -m cms.bin.xtm_translations create --name "Dummy Project" --desc "https://gitlab.com/eyeo/websites/cms/issues/12" --client-id 12449 --ref-id eyeo/websites/cms#12 --workflow-id 2926 .
2. Checkout German translations in editor:

Observed behavior

Instead of using the existing translation for string1 the source string is used.

Expected behavior

The existing translation for string1 should be used.