We have defined a special handling of derived types in dynamic expression edm:Record:

14.5.14 Expression edm:Record
The edm:Record expression enables a new entity type or complex type instance to be constructed.
A record expression contains zero or more edm:PropertyValue elements. For each single-valued property of the record construct's type that is neither nullable nor specifies a default value an edm:PropertyValue child element MUST be provided. For derived types this rule applies only to properties directly defined by the derived type. For collection-valued properties the absence of an edm:PropertyValue child element is equivalent to specifying a child element with an empty collection as its value.

For me this special handling of the derived types looks superfluous.