ulimit can be enlarged, but not reduced back to below original

Last login: Sat May 19 15:50:25 2018 from 192.168.0.130
~$ ulimit -u 5000
~$ ulimit -u 1000
~$ ulimit -u 10000
-bash: ulimit: max user processes: cannot modify limit: Operation not permitted
~$ ulimit -u 8000
-bash: ulimit: max user processes: cannot modify limit: Operation not permitted
~$ ulimit -u 7000
-bash: ulimit: max user processes: cannot modify limit: Operation not permitted
~$ ulimit -u 6000
-bash: ulimit: max user processes: cannot modify limit: Operation not permitted
~$ ulimit -u 5000
-bash: ulimit: max user processes: cannot modify limit: Operation not permitted
~$ ulimit -u 4000
-bash: ulimit: max user processes: cannot modify limit: Operation not permitted

~$ cat /etc/security/limits.conf | grep nproc | grep -v "^#"
* soft nproc 20480
* hard nproc 20480

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: bash 4.4.18-2ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7
Architecture: amd64
Date: Mon May 21 07:12:11 2018
InstallationDate: Installed on 2018-04-06 (44 days ago)
InstallationMedia: Ubuntu-Server 18.04 LTS "Bionic Beaver" - Alpha amd64 (20180402)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=C.UTF-8
 SHELL=/bin/bash
SourcePackage: bash
UpgradeStatus: No upgrade log present (probably fresh install)