Softwareupdate -L - no updates found

Kindly try the below command for updates
softwareupdate -i -a

Have done this and nothing found. I know I can download the package manual, but would like to test something with the downloaded version from software update

Try

sudo softwareupdate --clear-catalog
sudo softwareupdate --ignore --reset-ignored
sudo softwareupdate --list

Are your Macs pointing to a Software Update Server (SUS) other than Apple's servers?

Run a softwareupdate -i and then tail the log -

tail /private/var/log/install.log should show you all the updates similar to this output

2019-01-23 16:09:55-06 testmachine softwareupdated[233]: 6 updates found: 041-14451 | macOS Installer Notification 2.0 041-19430 | XProtectPlistConfigData 2101 041-26667 | MRTConfigData 1.38 041-31485 | Gatekeeper Configuration Data 160 041-31490(R) | Security Update 2019-001 10.13.6 (deferred) zzzz041-30443 | iTunes 12.8.2

Look for "deferred" next to any updates.

Fix: In your configuration profile check Restrictions payload - Functionality - at the bottom "Defer software updates for x days (macOS 10.13.4 or later)" - uncheck - save and redeploy config profile

It's a long shot, but if the device happens to be a laptop and it's encrypting, I believe it will not see any major macOS updates. Wait until encryption completes and you should be able to see the update.

I've seen odd behavior with software updates recently.

In one case the system with 10.14.2 didn't see 10.14.3 until a restart occurred. In another I had a system that saw 10.14.3 but would not start the update process until a restart was completed first. In both of those cases the updates did not show up in the App Store. I've also had several systems with High Sierra that would repeatedly want to apply security update 003.

On top of this I'm pretty sure it was security update 003 that was causing some of the random crashes on restart.

Sorry for the venting but software updates as of late have been pretty frustrating.