Seems that it was partially fixed in
https://bugzilla.opensuse.org/show_bug.cgi?id=926844 and does not happen for
zypper run, but happens when PackageKit is involved. Some users reported
thousands of directories
(https://forums.opensuse.org/showthread.php/543112-All-versions-There-are-over-500-000-zypp-directories-in-var-tmp-a-fix).
I do see 375 directories on Leap 15.1 where PackageKit is active by default. It
is trivial to trigger it, see below:

bor@10:~> ll -rt /var/tmp
total 0
drwx------ 1 root root  84 Jul 12 11:57 zypp.2qQWUv
drwx------ 1 root root 102 Jul 15 08:10 zypp.pF02Aq
drwx------ 1 root root  84 Jul 15 08:10 zypp.embiFR
drwx------ 1 root root  84 Jul 15 08:13 zypp.SOM6cZ
drwx------ 1 root root  84 Jul 15 08:18 zypp.YYGWx3
drwx------ 1 root root  84 Jul 18 10:17 zypp.l2IcuV
drwx------ 1 root root  84 Jul 18 10:18 zypp.00CBN3
drwx------ 1 root root 102 Jul 19 23:06 zypp.HQRPnb
drwx------ 1 root root  84 Jul 21 22:08 zypp.oyuf4R
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-ModemManager.service-POYDk8
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-chronyd.service-DGCnln
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-rtkit-daemon.service-V5IKvd
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-upower.service-Abw43c
bor@10:~> ll -rt /var/tmp/zypp.oyuf4R/
ls: cannot open directory '/var/tmp/zypp.oyuf4R/': Permission denied
bor@10:~> sudo ls -l -rt /var/tmp/zypp.oyuf4R/
[sudo] password for root: 
total 0
drwx------ 1 root root   0 Jul 21 22:08 zypp-general-krArO6IS
drwx------ 1 root root 102 Jul 21 22:08 zypp-trusted-kr2rXFoS
bor@10:~> sudo zypper patch
Retrieving repository 'Main Update Repository' metadata
..................[done]
Building repository 'Main Update Repository' cache
.......................[done]
Loading repository data...
Reading installed packages...
Resolving package dependencies...

The following 7 NEW packages are going to be installed:
  libjack0 libvulkan1 pipewire pipewire-modules pipewire-spa-plugins-0_2
  pipewire-spa-tools pipewire-tools

The following 9 NEW patches are going to be installed:
  openSUSE-2020-1182 openSUSE-2020-1189 openSUSE-2020-1190 openSUSE-2020-1191
  openSUSE-2020-1201 openSUSE-2020-1202 openSUSE-2020-1204 openSUSE-2020-1205
  openSUSE-2020-1206

The following 15 packages are going to be upgraded:
  chromium gnome-control-center-user-faces java-11-openjdk
  java-11-openjdk-headless libpipewire-0_3-0 libstorage-ng1 libstorage-ng-lang
  libstorage-ng-ruby libX11-data MozillaFirefox
  MozillaFirefox-translations-common MozillaThunderbird perl-XML-Twig xen-libs
  yast2-storage-ng

15 packages to upgrade, 7 new.
Overall download size: 228.5 MiB. Already cached: 0 B. After the operation,
additional 4.9 MiB will be used.
Continue? [y/n/v/...? shows all options] (y): n
bor@10:~> sudo ls -l -rt /var/tmp/zypp.oyuf4R/
total 0
drwx------ 1 root root   0 Jul 21 22:08 zypp-general-krArO6IS
drwx------ 1 root root 102 Jul 21 22:08 zypp-trusted-kr2rXFoS
bor@10:~> sudo ls -l -rt /var/tmp/
total 0
drwx------ 1 root root  84 Jul 12 11:57 zypp.2qQWUv
drwx------ 1 root root 102 Jul 15 08:10 zypp.pF02Aq
drwx------ 1 root root  84 Jul 15 08:10 zypp.embiFR
drwx------ 1 root root  84 Jul 15 08:13 zypp.SOM6cZ
drwx------ 1 root root  84 Jul 15 08:18 zypp.YYGWx3
drwx------ 1 root root  84 Jul 18 10:17 zypp.l2IcuV
drwx------ 1 root root  84 Jul 18 10:18 zypp.00CBN3
drwx------ 1 root root 102 Jul 19 23:06 zypp.HQRPnb
drwx------ 1 root root  84 Jul 21 22:08 zypp.oyuf4R
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-ModemManager.service-POYDk8
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-chronyd.service-DGCnln
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-rtkit-daemon.service-V5IKvd
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-upower.service-Abw43c
bor@10:~> pkcon refresh
Refreshing cache              [=========================]         
Starting                      [=========================]         
Refreshing software list      [=========================]         
Finished                      [=========================]         
bor@10:~> sudo ls -l -rt /var/tmp/
total 0
drwx------ 1 root root  84 Jul 12 11:57 zypp.2qQWUv
drwx------ 1 root root 102 Jul 15 08:10 zypp.pF02Aq
drwx------ 1 root root  84 Jul 15 08:10 zypp.embiFR
drwx------ 1 root root  84 Jul 15 08:13 zypp.SOM6cZ
drwx------ 1 root root  84 Jul 15 08:18 zypp.YYGWx3
drwx------ 1 root root  84 Jul 18 10:17 zypp.l2IcuV
drwx------ 1 root root  84 Jul 18 10:18 zypp.00CBN3
drwx------ 1 root root 102 Jul 19 23:06 zypp.HQRPnb
drwx------ 1 root root  84 Jul 21 22:08 zypp.oyuf4R
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-ModemManager.service-POYDk8
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-chronyd.service-DGCnln
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-rtkit-daemon.service-V5IKvd
drwx------ 1 root root   6 Aug 15 09:19
systemd-private-f39ae2fbdba44b69a28ba88d09a2ce6c-upower.service-Abw43c
drwx------ 1 root root  84 Aug 15 09:22 zypp.U5zkCH
bor@10:~>