But the Content-Range header and the Content-Length header both indicated the "expected" number of bytes e.g. the number of bytes that would have been returned if the server had given a 206 or a 200, not the truncated number of bytes that the response actually contained. Is that expected?

The latest response from the Chromium team (https://issues.chromium.org/issues/390229583#comment20) seems to take a different approach from your comment, and says that you should think of it as a streaming response where the connection failed partway through, which feels reasonable to me, except for the fact that `await`ing the response doesn't seem to trigger any errors: https://issues.chromium.org/issues/390229583#comment21