Can't Upload IAP Item Review Screenshots via the API

I'm uploading IAP Item review screenshots via the App Store Connect API. I create the request, then receive the upload operations. I perform the upload, making sure to populate the headers, and put the data in a buffer in the body of the PUT request.

https://developer.apple.com/documentation/appstoreconnectapi/create_an_in-app_purchase_review_screenshot

When that upload API call returns, it gives me back an empty buffer. Nothing I do seems to alter the buffer, including intentionally sending incorrect data as a test. Is this intended behavior? What does the empty buffer mean?

https://developer.apple.com/documentation/appstoreconnectapi/commit_a_review_screenshot_for_an_in-app_purchase

Then, without knowing whether my upload has succeeded, I try to commit the upload. That call always returns an unexpected internal server error. I suspect this is because the upload didn't actually succeed, and that the screenshot is still empty, but I have no way of knowing why. There's no documentation page for the actual upload API call, as far as I know. Here's the error.

{
      status: '500',
      code: 'UNEXPECTED_ERROR',
      title: 'An unexpected error occurred.',
      detail: 'An unexpected error occurred on the server side. If this issuue continues, contact us at https://developer.apple.com/contact/.'
    }

How do you upload IAP screenshots?