The Content-Type header did not specify encoding.
When test with Service Studio, it will consider it as UTF-8.
But Unfortunately at runtime, Platform will consider it as ASCII instead of UTF-8.
So you have two options.
1. Change REST Service to return Content-Type with encoding as following
Content-Type: application/json; charset=utf-8
2. If you are not allowed to Change REST Service, you can use OnBeforeRequest of Consume REST.
In CustomizedRequest, there are RequestBinary and RequestText.
You need convert RequestBinary to Text with UTF-8, ant then assign it to RequestText

Regards
Wei