Ubuntu server doesn't keep a static ip address

I have a problem on a pc that has ubuntu server installed (v. 18.04) that needs to connect as a client to a different server that has softether vpn installed. My client needs to have a static ip address, softether has set the dhcp server to assign ip addresses between 10.10.10.10 to 10.10.12.255.

The network interface of my ubuntu client is as follows:

network:
    version: 2
    renderer: networkd
    ethernets:
        vpn_interface:
            dhcp4: no
            addresses:
            - 10.10.10.2/16
            gateway4: 10.10.1.1
            nameservers:
                addresses:
                - 8.8.8.8
                - 8.8.4.4

The configuration is correct, and the client is correctly connected to the vpn, in fact when I run the sudo netplan apply command, the correct ip address is assigned:

However, every 15 minutes circa, the client is assigned a different ip address within the dhcp range (cat /var/log/syslog | grep -Ei 'dhcp'):

Nov 22 09:16:34 server dhclient[12640]: DHCPREQUEST of 10.10.10.98 on vpn_interface to 255.255.255.255 port 67 (xid=0x3bb4e04)
Nov 22 09:16:34 server dhclient[12640]: DHCPOFFER of 10.10.10.98 from 10.10.1.1
Nov 22 09:16:34 server dhclient[12640]: DHCPACK of 10.10.10.98 from 10.10.1.1
Nov 22 09:32:16 server dhclient[6582]: DHCPREQUEST of 10.10.10.98 on vpn_interface to 10.10.1.1 port 67 (xid=0x27fe6d20)
Nov 22 09:32:16 server dhclient[6582]: DHCPNAK from 10.10.1.1 (xid=0x206dfe27)
Nov 22 09:32:17 server dhclient[6582]: DHCPDISCOVER on vpn_interface to 255.255.255.255 port 67 interval 3 (xid=0xb4766d44)
Nov 22 09:32:17 server dhclient[6582]: DHCPREQUEST of 10.10.10.98 on vpn_interface to 255.255.255.255 port 67 (xid=0x446d76b4)
Nov 22 09:32:17 server dhclient[6582]: DHCPOFFER of 10.10.10.98 from 10.10.1.1
Nov 22 09:32:17 server dhclient[6582]: DHCPACK of 10.10.10.98 from 10.10.1.1
Nov 22 09:47:45 server dhclient[5809]: DHCPREQUEST of 10.10.10.98 on vpn_interface to 10.10.1.1 port 67 (xid=0x38481c28)
Nov 22 09:47:46 server dhclient[5809]: DHCPACK of 10.10.10.98 from 10.10.1.1
Nov 22 09:51:53 server dhclient[5971]: DHCPREQUEST of 10.10.10.98 on vpn_interface to 10.10.1.1 port 67 (xid=0x784c44e0)
Nov 22 09:51:53 server dhclient[5971]: DHCPACK of 10.10.10.98 from 10.10.1.1
Nov 22 10:05:36 server dhclient[12640]: DHCPREQUEST of 10.10.10.98 on vpn_interface to 10.10.1.1 port 67 (xid=0x3bb4e04)
Nov 22 10:05:36 server dhclient[12640]: DHCPNAK from 10.10.1.1 (xid=0x44ebb03)
Nov 22 10:05:37 server dhclient[12640]: DHCPDISCOVER on vpn_interface to 255.255.255.255 port 67 interval 3 (xid=0xe85f2113)
Nov 22 10:05:37 server dhclient[12640]: DHCPREQUEST of 10.10.10.98 on vpn_interface to 255.255.255.255 port 67 (xid=0x13215fe8)
Nov 22 10:05:37 server dhclient[12640]: DHCPOFFER of 10.10.10.98 from 10.10.1.1
Nov 22 10:05:37 server dhclient[12640]: DHCPACK of 10.10.10.98 from 10.10.1.1

How can i solve?

Softehter vpn version on client: 4.37

Softehter vpn version on server: 4.37

After the ip address has changed, if I run the sudo netplan apply command again, the client is configured correctly again