ProtoEnum attribute usage for renaming conflicting default values

I am attempting to generate c++ protobuf classes from a large c# .net codebase. The enumeration 'NotSet' is used as a default value in many enum declarations throughout, so I have used the ProtoEnum attribute to provide a different name for exporting where conflicts have arisen. However I have found that where my exported .proto file has classes with one of these enums as a member its default value still uses the orginal name, and protoc then fails to produce C++ files without correction.

Using the following example,

[DataContract]
[ProtoContract]
public enum MyEnum
{
    [EnumMember]
    [ProtoEnum(Name = "MyEnum_NotSet", Value=0)]
    NotSet,

    SomeValue,   
 
    AndSoOn
}

In a class I need to export the enumeration appears as a member I wish to export.

[ProtoContract]
public class MyClass
{
    [DataMember]
    [ProtoMember(1)]
    private MyEnum m_myEnum;
}

However in the exported .proto file, this class ends up with the original enum name as its default value...

syntax = "proto2";

enum MyEnum {
   MyEnum_NotSet = 0;
   SomeValue= 1;
   AndSoOn= 2;
}

message MyClass{
   optional MyEnum m_myEnum= 1 [default = NotSet];
}

Using protoc to generate c++ files from this fails - Enum type "MyEnum" has no value named "NotSet".

Is there some step I am missing here?