S3 multipart uploads failing in Safari

Using latest version of Uppy (and have tried with previous versions - same behavior) - S3 Multipart plugin, attempting to upload a local file to a bucket (so just getting the multipart links from companion and going straight to the bucket)

Upload fails. Safari console says:

[Uppy] Unknown Error
[Error] Preflight response is not successful

Indeed, it looks like the uppy client successfully gets the links to startup the transfer from companion and then the initial xhr request to amazon fails.

Companion has no interesting error output.

Is this a known Safari issue? Uploads via this method work perfectly fine in Chrome, Firefox and Opera.

This is Safari Version 13.1.2 (14609. in Mojave.

This is also happening in Safari on Catalina. Any ideas? The first thing that looks off in the Safari console is that the normal preflight OPTIONS don’t appear to be always happening. I suspect there’s an issue with Uppy and Safari’s implementation of fetch.