This seems to be working in 4.3.11 that is in xenial:
root@xenial-samba-dynamic-include:~# testparm -s
Load smb config files from /etc/samba/smb.conf
rlimit_max: increasing rlimit_max (1024) to minimum Windows limit (16384)
WARNING: The "syslog" option is deprecated
Can't find include file /etc/samba/smb.conf.
Loaded services file OK.
WARNING: The 'netbios name' is too long (max. 15 chars).

Server role: ROLE_STANDALONE

# Global parameters
[global]
	server string = %h server (Samba, Ubuntu)
	server role = standalone server
	map to guest = Bad User
	obey pam restrictions = Yes
	pam password change = Yes
	passwd program = /usr/bin/passwd %u
	passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
	unix password sync = Yes
	syslog = 0
	log file = /var/log/samba/log.%m
	max log size = 1000
	dns proxy = No
	usershare allow guests = Yes
	panic action = /usr/share/samba/panic-action %d
	idmap config * : backend = tdb
	include = /etc/samba/smb.conf.
root@xenial-samba-dynamic-include:~# echo $?
0

Notice how it didn't expand %U, and the include file was /etc/samba/smb.conf. (ending dot). Restart works:
root@xenial-samba-dynamic-include:~# systemctl restart smbd nmbd
root@xenial-samba-dynamic-include:~# systemctl status smbd nmbd
● smbd.service - LSB: start Samba SMB/CIFS daemon (smbd)
   Loaded: loaded (/etc/init.d/smbd; bad; vendor preset: enabled)
   Active: active (running) since Tue 2018-07-17 19:12:51 UTC; 3s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 2983 ExecStop=/etc/init.d/smbd stop (code=exited, status=0/SUCCESS)
  Process: 3012 ExecStart=/etc/init.d/smbd start (code=exited, status=0/SUCCESS)
    Tasks: 3
   Memory: 7.1M
      CPU: 127ms
   CGroup: /system.slice/smbd.service
           ├─3023 /usr/sbin/smbd -D
           ├─3024 /usr/sbin/smbd -D
           └─3026 /usr/sbin/smbd -D

Jul 17 19:12:51 xenial-samba-dynamic-include systemd[1]: Starting LSB: start Samba SMB/CIFS daemon (smbd)...
Jul 17 19:12:51 xenial-samba-dynamic-include smbd[3012]:  * Starting SMB/CIFS daemon smbd
Jul 17 19:12:51 xenial-samba-dynamic-include smbd[3012]:    ...done.
Jul 17 19:12:51 xenial-samba-dynamic-include systemd[1]: Started LSB: start Samba SMB/CIFS daemon (smbd).

● nmbd.service - LSB: start Samba NetBIOS nameserver (nmbd)
   Loaded: loaded (/etc/init.d/nmbd; bad; vendor preset: enabled)
   Active: active (running) since Tue 2018-07-17 19:12:51 UTC; 3s ago
     Docs: man:systemd-sysv-generator(8)
  Process: 2990 ExecStop=/etc/init.d/nmbd stop (code=exited, status=0/SUCCESS)
  Process: 2998 ExecStart=/etc/init.d/nmbd start (code=exited, status=0/SUCCESS)
    Tasks: 1
   Memory: 3.7M
      CPU: 127ms
   CGroup: /system.slice/nmbd.service
           └─3011 /usr/sbin/nmbd -D

Jul 17 19:12:51 xenial-samba-dynamic-include systemd[1]: Starting LSB: start Samba NetBIOS nameserver (nmbd)...
Jul 17 19:12:51 xenial-samba-dynamic-include nmbd[2998]:  * Starting NetBIOS name server nmbd
Jul 17 19:12:51 xenial-samba-dynamic-include nmbd[2998]:    ...done.
Jul 17 19:12:51 xenial-samba-dynamic-include systemd[1]: Started LSB: start Samba NetBIOS nameserver (nmbd).

Are you also seeing this behavior now?