I experience this same behaviour using lightdm + KDE plasma.
I've also tested lightdm + unity which did not trigger this behaviour.
This install uses local passwd/shadow/group files.
Both tests were after a fresh boot.

harm@harm-XPS-13-9360:~$ lsb_release -a; cat /proc/version
No LSB modules are available.                                                                                                                                                                                                                     
Distributor ID: Ubuntu                                                                                                                                                                                                                            
Description:    Ubuntu 18.04.1 LTS                                                                                                                                                                                                                
Release:        18.04                                                                                                                                                                                                                             
Codename:       bionic                                                                                                                                                                                                                            
Linux version 4.15.0-29-generic (buildd@lgw01-amd64-057) (gcc version 7.3.0 (Ubuntu 7.3.0-16ubuntu3)) #31-Ubuntu SMP Tue Jul 17 15:39:52 UTC 2018 

harm@harm-XPS-13-9360:~$ groups; groups $(whoami)
harm                                                                                                                                                                                                                                              
harm : harm adm dialout cdrom sudo dip plugdev netdev lpadmin sambashare libvirt docker

harm@harm-XPS-13-9360:~$ id; id $(whoami)
uid=1000(harm) gid=1000(harm) groups=1000(harm)                                                                                                                                                                                                   
uid=1000(harm) gid=1000(harm) groups=1000(harm),4(adm),20(dialout),24(cdrom),27(sudo),30(dip),46(plugdev),109(netdev),113(lpadmin),128(sambashare),135(libvirt),999(docker)

harm@harm-XPS-13-9360:~$ cat /etc/nsswitch.conf 
# /etc/nsswitch.conf                                                                                                                                                                                                                              
#                                                                                                                                                                                                                                                 
# Example configuration of GNU Name Service Switch functionality.                                                                                                                                                                                 
# If you have the `glibc-doc-reference' and `info' packages installed, try:                                                                                                                                                                       
# `info libc "Name Service Switch"' for information about this file.                                                                                                                                                                              
                                                                                                                                                                                                                                                  
passwd:         compat systemd                                                                                                                                                                                                                    
group:          compat systemd                                                                                                                                                                                                                    
shadow:         compat                                                                                                                                                                                                                            
gshadow:        files                                                                                                                                                                                                                             
                                                                                                                                                                                                                                                  
hosts:          files mymachines resolve [!UNAVAIL=return] dns myhostname                                                                                                                                                                         
networks:       files                                                                                                                                                                                                                             

protocols:      db files
services:       db files
ethers:         db files
rpc:            db files

netgroup:       nis