Dear Sir/Madam,
Thanks for your reply.
After checking the volume status, we suppose that it might be a known situation in the 16TB limited NAS model in the current DSM version.
We have expanded the volume manually successfully, now the volume size is 16TB. We will improve this issue in the later DSM version, thanks for your reporting, and sorry for any inconvenience to you.
I hope the answer helps and is clear.
Please kindly advise if you have any questions or further inquiry and I'm more than willing to help
Many thanks and wish you have a wonderful day!
Best Regards,
XXXXXXX
Technical Support Engineer / Synology Inc.