Serialization function never called if WithNetDeltaSerializer = true in RPC

Development Programming & Scripting
1

hello, epic
I have a struct defined with custom serialization function defined:

USTRUCT(BlueprintType)
struct FMyMJGamePusherPointersCpp
{
    GENERATED_USTRUCT_BODY()

    bool Serialize(FArchive& Ar);
    bool NetSerialize(FArchive& Ar, class UPackageMap* Map, bool& bOutSuccess);
    bool NetDeltaSerialize(FNetDeltaSerializeInfo & DeltaParms);

}

template<>
struct TStructOpsTypeTraits<FMyMJGamePusherPointersCpp> : public TStructOpsTypeTraitsBase2<FMyMJGamePusherPointersCpp>
{
    enum
    {

        WithSerializer = true,
        WithNetSerializer = true,
        //WithNetDeltaSerializer = true,  //If true, none of serialize funtion got called during RPC
    };
};


UCLASS(BlueprintType, Blueprintable)
class MYONLINECARDGAME_API AMyTestActorBaseCpp : public AActor
{
    GENERATED_BODY()

public:
    UFUNCTION(BlueprintCallable, Client, unreliable)
    void testRPCWithPusherPointers(const FMyMJGamePusherPointersCpp &pusherPointers);
}

And I got a problem that: if WithNetDeltaSerializer = true, no cunstom serialization function got called during the RPC call, so my question is, why that happens? is NetDeltaSerialize() forbidden in struct define?