Bug: Uploading content to new Document with `createDocWithContent` creates empty document

[MaSp005]

Hi, I am attempting to offer users to upload a locally created document to a shared document:

const yDoc = new Y.Doc();
const yMap = yDoc.getMap('crazyWall');
const unbind = bind<any>(WallProxy /* valtio proxy storing the JSON document */, yMap);
// I did test this, binding late does populate the empty storage with content from the other,
// printing the below buffer shows that it changes with differing input.

const state = Y.encodeStateAsUpdateV2(yDoc);
const buffer = Buffer.from(state);

return (await fetch("/api/newdoc", {
  method: "POST",
  body: buffer
}).then(r => r.json())).docId;

(See https://github.com/MaSp005/crazywall/blob/f10831b51944dd1dfed6c4c7f96e99f3fe0c6f1a/src/contexts/globalContext.ts#L276-L291)

The server will then create the document with the uploaded content:

const body = await request.arrayBuffer();
const content = new Uint8Array(body);
// This stores the same bytes as the buffer on the client.
const { docId } = await yManager.createDocWithContent(content);

(See https://github.com/MaSp005/crazywall/blob/f10831b51944dd1dfed6c4c7f96e99f3fe0c6f1a/src/index.ts#L70-L94)

---

Expected behaviour: Viewing the created document shows the exact state as uploaded before.

Observed behaviour: Visiting the created document afterwards shows an entirely empty document.

I attempted to step through the y-sweet server, and the update from the backend does reach DocWithSyncKv#apply_update, but I couldn't trace it further.

Is this an issue with my usage of the library or a bug with the server? Thanks!