Thanks, this fixed it for me.
I applied the workaround directly in app.yml via after_bundle_exec, pinned aws-sdk-s3 to 1.177.0 and aws-sdk-core to 3.215, then rebuilt the container. After that, manual backups to Cloudflare R2 succeeded again, and the previously failing browser uploads started working again as well.
In my case the failures were showing up as multipart upload failed: undefined method 'downcase' for nil on aws-sdk-s3 1.182.0.
Appreciate the workaround.