Customer is using AM as their IdP, replacing a legacy identity provider. Their legacy provider has the ID attribute defined in the EntityDecsriptor field when exchanging metadata files with the SP, as below
<ns2:EntityDescriptor xmlns:ns2="urn:oasis:names:tc:SAML:2.0:metadata" xmlns="http://www.w3.org/2000/09/xmldsig#" entityID="xxxxx" ID="_39636130-3063-3739-6235-363237353732">
When the metadata file is signed the SP is expecting the ID attribute to be present. Although the SAML specification states the field is optional
ID [*Optional*] - a document-unique identifier for the element, typically used as a reference point when signing.
The customer wishes to have this added and they're unable to use the entityID attribute
Please also refer to OPENAM-14309