Instead of going for the PipeTransform, which wasn't working properly for me, I ended up removing the @Type(() => YourClass) from the property and added a Tranform:

import { plainToInstance } from 'class-transformer';

class YourClass {
  @IsObject()
  @Transform(({ value }) =>
    plainToInstance<NestedClass, unknown>(
      NestedClass,
      typeof value === 'string' ? JSON.parse(value) : value,
    ),
  )
  @ValidateNested()
  property: NestedClass;
}