Spring controller Validation not working on HashSet of Objects

Question

I'm having trouble getting the @Valid annotation to fire in my Spring Controller.

Basically I have the following class:

import lombok.Getter;
import lombok.Setter;

import javax.validation.constraints.NotBlank;

@Getter
@Setter
public class PlayListDTO {

    @NotBlank(message = "playListName is required")
    private String playListName;
    private String playListDescription;
    private String playListUrl;
    private String playListImageUrl;
}

And the following controller:

   @PostMapping(path = "")
    public PlayListUGC addPlayList(@RequestBody HashSet<@Valid PlayListDTO> playListDTO,
                                   Authentication authentication,
                                   @RequestHeader(name = "Authorization") String token) {
        logger.info("Received Request to create new PlayList from User {}", authentication.getName());
        return playListService.savePlayList(playListDTO, authentication.getName());
    }

However if Post the following JSON the validation is not getting triggered:

[
  {
    "playListName":"",
    "playListDescription":"200",
    "playListUrl":"1234",
    "playListImageUrl": "playListImageUrl1"
  },
  {
    "playListName":"asdadsad",
    "playListDescription":"500",
    "playListUrl":"1235",
    "playListImageUrl": "playListImageUrl"
  }
]

Now, if I change my controller to take a RequestBody of just PlayListDTO the validation works i.e.

    @Operation(summary = "Allows a logged in user to add a playlist")
    @PostMapping(path = "")
    public PlayListUGC addPlayList(@RequestBody @Valid PlayListDTO playListDTO,
                                   Authentication authentication,
                                   @RequestHeader(name = "Authorization") String token) {
        logger.info("Received Request to create new PlayList from User {}", authentication.getName());
        return playListService.savePlayList(playListDTO, authentication.getName());
    } 

Can anyone explain to me why the validation does not work when its on a HashSet collection?

Thanks