Why isn't it possible for class-validator to validate @Body() of union type?

Question

register-user.dto.ts:

export default class RegisterUserDTO {
  @IsNotEmpty()
  @IsString()
  firstName: string;
  @IsNotEmpty()
  @IsString()
  lastName: string;
  @IsNotEmpty()
  @IsString()
  @IsEmail()
  email: string;
}

student-register.dto.ts:

export default class StudentRegisterDTO extends RegisterUserDTO {
  @IsISO8601()
  @IsOptional()
  dateOfBirth?: string;
  @IsString()
  @IsOptional()
  address?: string;
  @IsNotEmpty()
  @IsEnum(UserRoles)
  role: UserRoles.STUDENT;
}

trainer-register.dto.ts:

export default class TrainerRegisterDto extends RegisterUserDTO {
  @IsNotEmpty()
  @IsString()
  specialization: string;
  @IsNotEmpty()
  @IsEnum(UserRoles)
  role: UserRoles.TRAINER;
}

nestjs controller:

@Post("register")
  async register(
    @Body() studentOrTrainerDTO: StudentRegisterDTO | TrainerRegisterDTO,
  ) {
    return await this.authService.register(studentOrTrainerDTO);
  }

So whenever I test this endpoint with Postman, class-validator validates nothing on wrong input, however if I test this on only one class type (either StudentRegisterDTO or TrainerRegisterDTO), class-validator works perfectly fine, which made me think it just won't work on union types, why is that and is there a solution to this?