404 for Polly mp3 generated into s3 bucket

I think this might be an issue with S3 Access rights/permissions, but I am lost as to how to proceed.

I create a mp3 file with this: $result_audio = $client->startSpeechSynthesisTask([ 'Engine' => 'standard', 'OutputFormat' => 'mp3', 'OutputS3BucketName' => 'BUCKET', 'Text' => $chunk_text, 'VoiceId' => $row_data['cv_service_id'] ]);

$s3KeyName = substr($result_audio['SynthesisTask']['OutputUri'], 43);

$s3->copyObject([ 'Bucket' => 'BUCKET', 'Key' => urlencode($new_id.'.mp3'), 'CopySource' => "BUCKET/".urlencode($s3KeyName), ]);

I replaced my bucket with BUCKET for this post.

I get a 404 not found on the copy object, even though I have double checked that the temp file is created. It is auto generated by Amazon Polly and I cannot change the way it is saved. It does not contain anything I think needs to be urlencoded, but I added that to make sure.

An example of the $s3KeyName: 3b7901d6-52af-44ce-9993-b2f805153e85.mp3

I have used the Policy simulator to check that the IAM user credentials have access: https://policysim.aws.amazon.com/home/index.jsp I checked the following permissions: S3 DeleteObject GetObject PutObject GetObjectTagging PutObjectTagging PutObject ListAllMyBuckets

s3express CreateSession

I have granted the IAM user full access to s3 and s3 express, but still nothing changes. I do have the bucket set to prevent public access with all options checked, but I don't think that should stop an authenticated IAM user?

There is no way to set the filename when creating an Amazon Polly mp3 file with startSpeechSynthesisTask, right?

Any guidance what I should do?