Looks like there may be an issue with the signature logic on your Lambda. Files smaller than 5 MiB are sent in a single request, non-chunked, with a policy file using the S3 HTML form upload endpoint. Files larger than 5 MiB are chunked by Fine Uploader and sent through the AWS Multipart Upload Rest API. You already know this, but I wanted to be complete in my explanation.
The non-chunked signatures are working flawlessly, but there appears to be an issue with larger/chunked files. I will take a look at your signing logic and let you know what I see. If you get to it first, please let me know.
Thanks for your work on this project. Very cool article, graphics, etc.