401 Unauthorized error when submitting multipart/form-data without multipartfile

I'm a beginner working on a team project and currently creating a "board" page in React + Spring Boot.

I'm really confused because I keep getting a 401 Unauthorized error when submitting a POST request to create a board with multipart/form-data.

• If I include a file, everything works fine.
• If I submit without a file, I get 401 INVALID_ACCESS_TOKEN from the backend.

I'm new to both Spring Security and like everything, and I'm working in a team, so I want to make this work properly for all cases.

Has anyone faced a similar issue or can explain why a multipart request without files might cause a JWT to fail authentication?

controller:

@PostMapping("/create")
public ResponseEntity<?> createBoard(
            @PathVariable("crewId") Long crewId,
            @RequestPart("title") String title,
            @RequestPart("content") String content,
            @RequestParam(value = "crewBoardFile", required = false) List<MultipartFile> crewBoardFile,
            @AuthenticationPrincipal MyUserDetails userDetails
) throws IOException {
    CrewBoardDto crewBoardDto = new CrewBoardDto();
    crewBoardDto.setTitle(title);
    crewBoardDto.setContent(content);
    crewBoardDto.setCrewBoardFile(crewBoardFile);

    Long loginUserId = userDetails.getMemberId();
    CrewBoardDto createBoard = crewBoardService.createBoard(crewId, crewBoardDto, loginUserId, crewBoardFile);

    return ResponseEntity.ok(createBoard);
}

service:

 @Override
public CrewBoardDto createBoard(Long crewId, CrewBoardDto crewBoardDto, Long loginUserId, List<MultipartFile> crewBoardFile) throws IOException {
    CrewEntity crewEntity = crewRepository.findById(crewId)
            .orElseThrow(() -> new IllegalArgumentException("x"));

    MemberEntity memberEntity = memberRepository.findById(loginUserId)
            .orElseThrow(() -> new IllegalArgumentException("x"));

    crewRepository.findByIdAndCrewMemberEntities_MemberEntity_Id(crewId, loginUserId)
            .orElseThrow(() -> new IllegalArgumentException("x"));

    crewBoardDto.setMemberId(memberEntity.getId());
    crewBoardDto.setCrewId(crewId);

    CrewBoardEntity crewBoardEntity = CrewBoardEntity.toCrewBoardEntity(crewBoardDto);
    
    CrewBoardEntity savedBoard = null;

    crewBoardFile = crewBoardDto.getCrewBoardFile();
    
    if (crewBoardFile == null || crewBoardFile.isEmpty() || crewBoardFile.get(0).isEmpty()) {
        savedBoard = crewBoardRepository.save(crewBoardEntity);            
    } else {
        List<CrewBoardImageEntity> savedImages = new ArrayList<>();
        // for (MultipartFile file : crewBoardDto.getCrewBoardFile()) {
            //     if (file != null && !file.isEmpty()) {
                //         String originalFileName = file.getOriginalFilename();
                
                //         String newFileName = awsS3Service.uploadFile(file);
                
                //         CrewBoardImageEntity boardImageEntity = CrewBoardImageEntity.toCrewBoardImageEntity(crewBoardEntity2, originalFileName, newFileName);
                
                //         crewBoardImageRepository.save(boardImageEntity);
                //         crewBoardRepository.save(crewBoardEntity);
                //     }
        // }
        for (MultipartFile boardFile : crewBoardFile) {
            if (boardFile != null && !boardFile.isEmpty()) {
                UUID uuid = UUID.randomUUID();
                String originalFileName = boardFile.getOriginalFilename();
                String newFileName = uuid + "_" + originalFileName;

                String filePath = "E:/full/upload/" + newFileName;
                File file = new File(filePath);

                boardFile.transferTo(file);
                
                CrewBoardImageEntity boardImageEntity = CrewBoardImageEntity.toCrewBoardImageEntity(crewBoardEntity, originalFileName, newFileName);
                CrewBoardImageEntity savedImage = crewBoardImageRepository.save(boardImageEntity);
                savedImages.add(savedImage);
            }
        }
        crewBoardEntity.setCrewBoardImageEntities(savedImages);
        savedBoard = crewBoardRepository.save(crewBoardEntity);            

    }
    return CrewBoardDto.toDto(savedBoard);
}

react:

react