$ sudo dnf upgrade tigervnc
Last metadata expiration check: 1:08:51 ago on Thu 26 Jul 2018 07:23:35 AM EDT.
Dependencies resolved.

 Problem: problem with installed package avahi-ui-tools-0.7-12.fc28.x86_64
  - package avahi-ui-tools-0.7-12.fc28.x86_64 requires vnc, but none of the providers can be installed
  - cannot install both tigervnc-1.9.0-1.fc28.x86_64 and tigervnc-1.8.0-10.fc28.x86_64
  - cannot install both tigervnc-1.8.0-7.fc28.x86_64 and tigervnc-1.9.0-1.fc28.x86_64
  - cannot install the best update candidate for package tigervnc-1.8.0-10.fc28.x86_64
=============================================================================================
 Package              Arch               Version                   Repository           Size
=============================================================================================
Skipping packages with conflicts:
(add '--best --allowerasing' to command line to force their upgrade):
 tigervnc             x86_64             1.8.0-7.fc28              fedora              255 k
 tigervnc             x86_64             1.9.0-1.fc28              updates             273 k

Transaction Summary
=============================================================================================
Skip  2 Packages

Nothing to do.
Complete!
$ sudo dnf upgrade tigervnc --best --allowerasing
Last metadata expiration check: 1:09:15 ago on Thu 26 Jul 2018 07:23:35 AM EDT.
Dependencies resolved.
=============================================================================================
 Package                  Arch             Version                   Repository         Size
=============================================================================================
Upgrading:
 tigervnc                 x86_64           1.9.0-1.fc28              updates           273 k
Removing dependent packages:
 avahi-ui-tools           x86_64           0.7-12.fc28               @fedora            63 k

Transaction Summary
=============================================================================================
Upgrade  1 Package
Remove   1 Package

Total download size: 273 k
Is this ok [y/N]: n
Operation aborted.
$ rpm -q --requires avahi-ui-tools
...
vnc
$ dnf repoquery --whatprovides vnc
Last metadata expiration check: 0:00:00 ago on Thu 26 Jul 2018 08:34:29 AM EDT.
tigervnc-0:1.8.0-7.fc28.x86_64
$ dnf repoquery --provides tigervnc-1.9.0-1.fc28.x86_64
Last metadata expiration check: 0:01:03 ago on Thu 26 Jul 2018 08:34:29 AM EDT.
application()
application(vncviewer.desktop)
tigervnc = 1.9.0-1.fc28
tigervnc(x86-64) = 1.9.0-1.fc28
$ dnf repoquery --whatrequires vnc
Last metadata expiration check: 0:06:58 ago on Thu 26 Jul 2018 08:34:29 AM EDT.
avahi-ui-tools-0:0.7-12.fc28.x86_64
gitso-0:0.6-17.fc28.noarch

avahi-ui-tools and gitso require "vnc" but the only package providing it is the older release of tigervnc.  The update seems to have removed the "vnc" "provides".