Ubuntu 18.04: Host name changes from FQDN to PQDN in DNS

We are running Ubuntu 18.04 LTS with networking configured by Netplan. The network has a Windows DHCP server and a Windows DNS server.

The Ubuntu 18.04 machine uses this Netplan cfg file to obtain a dynamic IP address:

$ cat /etc/cloud/cloud.cfg.d/50-curtin-networking.cfg
network:
   ethernets:
     ens4:
     addresses: []
     dhcp4: true
     version: 2

The hostname and domain name are:

$ hostname
ubuntusim01
$ dnsdomainname
uktm.<snip>.com

After reboot the FQDN in DNS is:

ubuntusim01.uktm.<snip>.com

which is correct. But the host name stored in DNS sometimes spontaneously changes to the PQDN:

ubuntusim01.

(interrogated by nslookup) and then other systems lose connectivity to the Ubuntu machine.

It is possible that this change happens when the DHCP lease expires, but we are not sure of that.

This problem does not occur with our Ubuntu 16.04 LTS servers (which use NetworkManager).

Is our configuration wrong in some way or could this be a bug in Netplan?