highend

Since you're talking about the OMV Armbian variant it is resizing but to 3.7GB by design (that's how @ryeaaron built all his OMV ARM images before and I followed his convention): https://github.com/armbian/build/blob/b0d4931a5bcba66288a4ff6180186699fefcb947/scripts/customize-image.sh.template#L111
 
The remaining capacity is there as /dev/mmcblk0p2 so you can do whatever you want with it (eg. share it with Samba for frequently used data to prevent spinning up sleeping disks or as a btrfs filesystem with maximum transparent file compression to store logs or whatever). If you want 'traditional' Armbian behaviour with rootfs resize to the maximum you would need to remove /root/.rootfs_resize prior to first boot.