Inspecting the body of a previous webhook with the body of a different webhook indicates that there are differences with regards to included field attributes.
Release : SAAS
Component : Webhooks
The payload will change dynamically depending on the fields present in the project. The following scenarios will affect field visibility within the payload
1) A work item's parent project. Since fields can be defined on a project-by-project basis, a work item that exists in one project may have a different set of fields than a work item in a different project.
2) Fields being added to a workspace. If a new field is added to a workspace, then there will be new field information added to the webhook body.
3) Fields being hidden in a workspace/project. When fields are hidden, then those fields will no longer be included in the webhook body.