If you can't work it out then write in to support and we can find out for you

if (md.val) {
  console.log("MD", md.val)
  await env.CONTENT_BUCKET.put(`${key}-md`, new ReadableStream(md.val))
}

if (md.val) {
  console.log("MD", md.val)
  await env.CONTENT_BUCKET.put(`${key}-md`, new ReadableStream(md.val))
}

while (true) {
  let foo = await reader.read();
  if (foo.value) {
  console.log(foo.value)
  let md = metadataExtractor.extract(foo.value)
  if (md.val) {
    let encoder = new TextEncoder()
    let strung = encoder.encode(JSON.stringify(md.val))
    console.log("MD", strung)
    console.log(strung.length)
    let {readable, writable} = new FixedLengthStream(strung.length)
    let writer = writable.getWriter()
    await writer.write(strung).then(e => console.log({e}))
    await env.CONTENT_BUCKET.put(`${key}-md`, readable)
    console.log("PUT METADATA IN R2")
  }
}

while (true) {
  let foo = await reader.read();
  if (foo.value) {
  console.log(foo.value)
  let md = metadataExtractor.extract(foo.value)
  if (md.val) {
    let encoder = new TextEncoder()
    let strung = encoder.encode(JSON.stringify(md.val))
    console.log("MD", strung)
    console.log(strung.length)
    let {readable, writable} = new FixedLengthStream(strung.length)
    let writer = writable.getWriter()
    await writer.write(strung).then(e => console.log({e}))
    await env.CONTENT_BUCKET.put(`${key}-md`, readable)
    console.log("PUT METADATA IN R2")
  }
}

A hanging Promise was canceled. This happens when the worker runtime is waiting for a Promise from JavaScript to resolve, but has detected that the Promise cannot possibly ever resolve because all code and events related to the Promise's I/O context have already finished.
A hanging Promise was canceled. This happens when the worker runtime is waiting for a Promise from JavaScript to resolve, but has detected that the Promise cannot possibly ever resolve because all code and events related to the Promise's I/O context have already finished.

A hanging Promise was canceled. This happens when the worker runtime is waiting for a Promise from JavaScript to resolve, but has detected that the Promise cannot possibly ever resolve because all code and events related to the Promise's I/O context have already finished.
A hanging Promise was canceled. This happens when the worker runtime is waiting for a Promise from JavaScript to resolve, but has detected that the Promise cannot possibly ever resolve because all code and events related to the Promise's I/O context have already finished.

writer.write(strung).then(() => {
  env.CONTENT_BUCKET.put(`${key}-md`, readable)
}))

writer.write(strung).then(() => {
  env.CONTENT_BUCKET.put(`${key}-md`, readable)
}))