Could you share the specific code you’re using to modify the response body? The [Object ReadableStream] string suggests that you are perhaps doing something like new Response(response.body + "new content"), thus stringifying response.body, which is a ReadableStream, not a string.
If you know for sure that the response is text (e.g., with if (response.headers.get("Content-Type").startsWith("text/")), you could use new Response(await response.text() + "new content").