When using the ServiceAssignmentCollection service with OData, you notice that this service only allows the update of the Status property.
SAP Cloud for Customer
At the moment, only the Status property is released to be updated on this service via OData.
This is the system expected behavior.
More details on the properties of this service can be found in https://help.sap.com/doc/d0f9ba822c08405da7d88174b304df84/CLOUD/en-US/docs/topics/00163E07C0211EE885FD43765C847BF9.html
OData, Service, Assignment, Status, Property, ServiceAssignmentCollection, Services, C4C, , KBA , LOD-CRM-RES , Resource Scheduling , How To