In section 11.4.3, Update an Entity, we are clear that non-updatable properties specified in PUT or PATCH are ignored:
"Key and other non-updatable properties, as well as dependent properties that are not tied to key properties of the principal entity, can be omitted from the request. If the request contains a value for one of these properties, the service MUST ignore that value when applying the update."
However, we also make general statements like:
"Collection properties and primitive properties provided in the payload MUST replace the value of the corresponding property in the entity or complex type"
"Services that support PUT MUST replace all values of structural properties with those specified in the request body"
We should clarify that these later statements apply to updatable structural properties.