Why is @Expose() decorator required to create new property fields?

[Krrish-raj]

Say I have the following class definition:

export class Filter {
    @IsEnum(FilterType)
    id: FilterType

    @IsString()
    /* This is not getting executed. **/
    @Transform(({ obj }) => LabelGenerator.getfilterTypeLabel(obj.id), { toClassOnly: true })
    label: string;

    @IsEnum(FilterSelectType)
    select: FilterSelectType;
}

I am using a plain object: {id: 'Sample', select: 'multi'} to create the class.

In the above case, @Transform decorator is not getting executed, but if I put @expose() decorator on top of that, it works as expected. Is this the expected behavior?