External HDD - File created from OSX is not visible in OpenWrt (Linux)

Question

I am using an external HDD as a media server. The HDD is formated as exFAT with GPT (from Windows).

I noticed that files created from OS X while the HDD was mounted on my Mac are not visible after mounting the disk on OpenWrt (router).

The files are not hidden as both du and ls -la return empty contents for the folder. No OS X attributes are set on the files, I checked with xattr. Journaling is not enabled on the HDD.

I mounted the HDD on OpenWrt with rw rights:

root@OpenWrt:/# mount | grep seagate
/dev/sda2 on /mnt/seagate type exfat (rw,relatime,fmask=0022,dmask=0022,namecase=1,errors=remount-ro)

fdisk does not show any errors:

root@OpenWrt:/# fdisk -l /dev/sda

Disk /dev/sda: 2.7 TiB, 3000592981504 bytes, 5860533167 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 4096 bytes
I/O size (minimum/optimal): 4096 bytes / 4096 bytes
Disklabel type: gpt
Disk identifier: 52E20666-389A-4415-BE39-501CEFF51221

Device      Start        End    Sectors  Size Type
/dev/sda2  411648 5860532223 5860120576  2.7T Microsoft basic data

And an ls and du on one of the folders:

root@OpenWrt:/# du -h /mnt/seagate/Work
1.0M    /mnt/seagate/Work

root@OpenWrt:/# ls -la  /mnt/seagate/Work
drwxr-xr-x    2 root     root       1048576 Mar  4 08:35 .
drwxr-xr-x   86 root     root       1048576 Mar  7 21:43 ..

Files inside are visible from OS X if I connect the HDD back to my Mac:

ninja:/ liana$ du -h /Volumes/Seagate\ 1/Work
8.4M    /Volumes/Seagate 1/Work

Output of diskutil list for the HDD on Mac:

ninja:/ liana$ diskutil list
/dev/disk2 (external, physical):
   #:      TYPE NAME                                  SIZE       IDENTIFIER
   0:      GUID_partition_scheme                      *3.0 TB     disk2
   1:      Microsoft Basic Data Seagate                3.0 TB     disk2s2

Edit:

Here are the Kernel version, OpenWrt version and kmod-fs-exfat version:

root@OpenWrt:~# uname -a
Linux OpenWrt 3.18.20 #1 Fri Sep 4 21:55:57 CEST 2015 mips GNU/Linux

root@OpenWrt:~# cat /etc/*release*
DISTRIB_ID='OpenWrt'
DISTRIB_RELEASE='15.05'
DISTRIB_REVISION='r46767'
DISTRIB_CODENAME='chaos_calmer'
DISTRIB_TARGET='ar71xx/generic'
DISTRIB_DESCRIPTION='OpenWrt Chaos Calmer 15.05'
DISTRIB_TAINTS=''

root@OpenWrt:~# opkg list-installed | grep exfat
kmod-fs-exfat - 3.18.20+git-20150301-1

All of the other kmod packages are 3.18.20.

I have been trying for the past couple of days to get them to display and I am running out of ideas. Any help would be much appreciated!