QNAP to QNAP Sync Job via VPN does full resync if the NAS is relocated

[oscarcahill]

OK, so we can sync fine via a VPN tunnel between two QNAP NASs once the first sync has completed. But, if I move the offsite target QNAP to a different offsite location, even though the static IP address is the same for both QNAPS as we are using a VPN, it does a FULL resync which can take days. Once that is finished, it's fine. But if I move it back to the first offsite location it does a full resync again! So what is going on and why does it do a full resync? Neither QNAP has a change in IP address.
Details: Source QNAP: TS-EC880U, target QNAP: TS-832A. All up to date with firmware etc. Job type: Sync. VPN tunnel is via two Sonicwall firewalls, one at each end.
This has been driving us nuts! Any help or insight would be fantastic.

[dolbyman]

So that means that no setting is changed, the NAS (IPs on both sides) and RSYNC settings remain on the same settings?

Also please always disclose 'up to date firmware' as currently QTS has two 'up to date' firmware releases (5.0 and 4.5)

[oscarcahill]

Correct Dolbyman. Both IP addresses remain the same and the RSYNC settings are unchanged.
Source QNAP: TS-EC880U Firmware 5.0.0.2131 (update is now available oops)
target QNAP: TS-832A firmware: QTS 5.0.1.2194 20221022