[UC] not able to copy files to vfat partition if partition size is less than 33M

[Summary]
when using ubuntu-image to build ubuntu core image, there will be an issue copying file to vfat partition if the partition size is less than 33M

[Error message]
env UBUNTU_STORE_AUTH_DATA_FILENAME=/home/vagrant/aristo/image-builds/store.auth ubuntu-image snap --debug -O out -w work --channel stable --snap=./mediatek-aiot-kernel_28.snap --snap=./snapd_19459.snap --snap=./mediatek-aiot_22-0.3.2_arm64.snap --snap=./core22_821.snap --disk-info buildstamp model.assert
[0] make_temporary_directories
[1] determine_output_directory
[2] prepare_image
WARNING: proceeding to download snaps ignoring validations, this default will change in the future. For now use --validation=enforce for validations to be taken into account, pass instead --validation=ignore to preserve current behavior going forward
WARNING: the kernel for the specified UC20+ model does not carry assertion max formats information, assuming possibly incorrectly the kernel revision can use the same formats as snapd
WARNING: "mediatek-aiot" installed from local snaps disconnected from a store cannot be refreshed subsequently!
Copying "./snapd_19459.snap" (snapd)
Copying "./mediatek-aiot-kernel_28.snap" (mediatek-aiot-kernel)
Copying "./core22_821.snap" (core22)
Copying "./mediatek-aiot_22-0.3.2_arm64.snap" (mediatek-aiot)
[3] load_gadget_yaml
[4] set_artifact_names
[5] populate_rootfs_contents
[6] generate_disk_info
[7] calculate_rootfs_size
[8] populate_bootfs_contents
[9] populate_prepare_partitions
Error: Error running mkfs with content: cannot populate vfat filesystem with contents: No directory slots

[sample gadget.yaml that have issue]
volumes:
  pc:
    schema: gpt
    bootloader: grub
    structure:
      - name: firmware
        filesystem: vfat
        type: 384E979B-EB76-435A-A3A6-1A071DBAD91D
        size: 32M
        content:
          - source: shimaa64.efi.signed
            target: EFI/BOOT/BOOTAA64.efi
      - name: ubuntu-seed
        role: system-seed
        filesystem: vfat
        type: EF,C12A7328-F81F-11D2-BA4B-00A0C93EC93B
        size: 1200M
        content:
          - source: $kernel:dtbs/dtbs/mediatek/
            target: FIRMWARE/mediatek/genio-350-evk/
      - name: ubuntu-boot
        role: system-boot
        filesystem: ext4
        type: 83,0FC63DAF-8483-4772-8E79-3D69D8477DE4
        size: 750M
      - name: ubuntu-save
        role: system-save
        filesystem: ext4
        type: 83,0FC63DAF-8483-4772-8E79-3D69D8477DE4
        size: 32M
      - name: ubuntu-data
        role: system-data
        filesystem: ext4
        type: 83,0FC63DAF-8483-4772-8E79-3D69D8477DE4
        size: 1G

[Other info]
- If the firmware partition filesystem type is changed to ext4, then there is no issue
- If the size of the firmware partition is greater than or equal to 33M, then there is no issue

[Other info - snap list]
$ snap list
Name Version Rev Tracking Publisher Notes
core18 20230530 2785 latest/stable canonical✓ base
core20 20230622 1974 latest/stable canonical✓ base
core22 20230703 817 latest/stable canonical✓ base
lxd 5.0.2-838e1b2 24322 5.0/stable/… canonical✓ -
snapcraft 7.5.0 9510 7.x/stable canonical✓ classic
snapd 2.59.5 19457 latest/stable canonical✓ snapd
ubuntu-image 3.0+snap4 500 latest/stable canonical✓ classic
yq v4.34.2 2219 latest/stable mikefarah -