Single Uploads Succeed; Multiparts Fail

I am using Uppy to upload to S3 with signed URLs using @uppy/aws-s3. I already have a server, so I am not using Transloadit. Sorry.

Single uploads work well, which suggest signing and CORS are OK. But when I try multipart uploads, all the preflight OPTIONS calls work, but the uploads themselves fail because they get canceled (not by me).

An example URL that gets canceled:,%20public&X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Date=20231218T223041Z&X-Amz-SignedHeaders=content-type;host&X-Amz-Expires=900&X-Amz-Credential=AKIAI4HGEBPUXDEMOS5Q/20231318/us-east-1/s3/aws4_request&X-Amz-Signature=7aaf9f1d5cb9ddc82809bd29f99718c9f03518df59d0b99c24abcc8fae21b14&partNumber=31&uploadId=4YUkxkeDD.uFWsTpbwsu69ZHXSc0YB7SS8h9ke2a6clj4nZq63D4LC6qxtoe1at7Y8gaJL_EeUBzED.zE7ORgcz8K4HZcQLgkve_wT.2lwD0yARUBeXAu0qpGwIjhAV4

You can see the partNumber and uploadId at the end.

Any ideas what is causing the cancelation?