Are PowerForm URL parameters case sensitive?

Yes. PowerForm URL parameters are case sensitive.

Example URLs

Works
https://demo.docusign.net/Member/PowerFormSigning.aspx?PowerFormId=1a2b3c4d-1f19-415a-aa43-0d14e5d3261c&Role_UserName=John%20Hancock&Role_Email=john.hancock@docusign.com

Fails
https://demo.docusign.net/Member/PowerFormSigning.aspx?PowerFormId=1a2b3c4d-1f19-415a-aa43-0d14e5d3261c&role_username=John%20Hancock&role_email=john.hancock@docusign.com

Related

Populate Custom Document Fields in a Web PowerForm - New DocuSign Experience