Can't migrate uploads to S3

Yes, they are in app.yml and I’ve rebuilt the app.
I’ve confirmed that newly uploaded files are going there.

Not quite. I’m using the following environmental variables:

  AWS_REQUEST_CHECKSUM_CALCULATION: WHEN_REQUIRED
  AWS_RESPONSE_CHECKSUM_VALIDATION: WHEN_REQUIRED

Those fixed not being able to upload new files after the settings change.