Missing message from decline callback

Any thoughts on why message is null? I entered text on the decline to sign.  I received the decline event notification.

Event:

11:05:02:031 USER_DEBUG [16]|DEBUG|ossCallback: OSSCallback:[documentId=null, message=, name=PACKAGE_DECLINE, packageId=z8dM5486HxkamDPcX9kZ6x6Ma0A=, sessionUser=c1d15155-6ddd-4c5a-881a-e2d0f1cb5cc0]

This is my rest resource class: 

@RestResource(urlMapping='/oss/callback')
global with sharing class OSSCallbackResource {
    @HttpPost
    global static void callback() {
        OSSCallback ossCallback = (OSSCallback)JSON.deserialize(RestContext.request.requestbody.tostring(), OSSCallback.class);
                
        OneSpan_Event__e platformEvent = new OneSpan_Event__e(
            Name__c = ossCallback.name,
            Package_ID__c = ossCallback.packageId,
            Session_User__c = ossCallback.sessionUser,
            Document_ID__c = ossCallback.documentId,
            Message__c = ossCallback.message
        );
        
        EventBus.publish(new List<OneSpan_Event__e>{platformEvent});
        System.debug('ossCallback: ' + ossCallback);
    }
    
    global class OSSCallback {
        global String name;
        global string sessionUser;
        global String packageId;
        global string message;
        global String documentId;
    }
}