After a few hours of try, I finally know what’s going wrong.
It seems that IDetailLayoutBuilder::GetProperty can only get the property handle of the property that will be shown in Detail View by default.
Therefore, EditAnywhere, VisibleAnywhere, or something like that should appear inside UPROPERTY().
After revision:
UCLASS()
class DETAILVIEW_TEST_API UMyDataAsset : public UDataAsset
{
GENERATED_BODY()
public:
UPROPERTY(EditAnywhere)
int Num1;
UPROPERTY(EditAnywhere, meta = (ClampMin = "0", ClampMax = "100"))
int Num2;
};