scary/unexpected output in adduser and deluser

On 16.04 images on Azure,
adduser and deluser are now printing to stderr 'sent invalidate(passwd)' and
'sent invalidate(group)'.

These are confusing at best.
See below. They do not seem harmful, but not wonderful.

I only see them on images of 16.04 on Azure. I cannot reproduce in
16.04 or 18.04 images on LXD, Openstack. I cannot reproduce in 18.04
on Azure.

smoser@ubuntu1:~$ sudo adduser guest
Adding user `guest' ...
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
Adding new group `guest' (1001) ...
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
sent invalidate(group) request, exiting
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
Adding new user `guest' (1001) with group `guest' ...
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
The home directory `/home/guest' already exists. Not copying from `/etc/skel'.
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
Changing the user information for guest
Enter the new value, or press ENTER for the default
        Full Name []: Foo Bar
        Room Number []:
        Work Phone []:
        Home Phone []:
        Other []:
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
sent invalidate(passwd) request, exiting
Is the information correct? [Y/n]
smoser@ubuntu1:~$ sudo deluser guest
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
Removing user `guest' ...
Warning: group `guest' has no more members.
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
sent invalidate(passwd) request, exiting
sent invalidate(group) request, exiting
Done.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: adduser 3.113+nmu3ubuntu4
ProcVersionSignature: User Name 4.13.0-1007.9-azure 4.13.13
Uname: Linux 4.13.0-1007-azure x86_64
ApportVersion: 2.20.1-0ubuntu2.15
Architecture: amd64
Date: Fri Feb 16 19:30:12 2018
PackageArchitecture: all
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: adduser
UpgradeStatus: No upgrade log present (probably fresh install)