13 minutes left after login

Greetings all,

We are having a rather odd thing happen on some, but not all, of our lab machines. These are in an academic setting, and are open labs for student use. The Macs are of various types, and are bound to our Active Directory system for authentication. They are still running Sierra, but we are in the process of upgrading them to Mojave as part of an overall updating of our build procedure (leaving monolithic behind, which was one of the reasons these were still on Sierra, and moving toward a Jamf Pro based build structure). I am using the following script, sent out via Apple Remote Desktop, to update the systems to Mojave in place, so that we can then "nuke and pave" them (using the --eraseinstall feature in phase 2) to get them into the Jamf system via Device Enrollment. All of the serial numbers have been added to Apple School Manager. Anyway, the script is:

# Phase one, upgrade in place to Mojave

/usr/bin/caffeinate -dimsu &
/bin/echo "caffeinate result: $?"

/usr/bin/curl -o /Users/Shared/Mojave.dmg http://XXXXX.XXXXX.nd.edu/utils/Mojave.dmg
/bin/echo "cURL result: $?"
/bin/sleep 5

/usr/bin/hdiutil attach /Users/Shared/Mojave.dmg -nobrowse -quiet -readonly
/bin/echo "hdiutil result: $?"
/bin/sleep 5

/Volumes/Mojave/Install\ macOS\ Mojave.app/Contents/Resources/startosinstall --applicationpath /Volumes/Mojave/Install\ macOS\ Mojave.app --agreetolicense --nointeraction &

After this completes, the login screen comes up, and everything LOOKS normal, but as soon as I log in with my local administrator account the system goes to the Apple with the progress bar underneath it with 13 minutes to go. It's always 13 minutes. Then, shortly after, whatever it is doing completes, and it goes to the (not sure what this is actually called) password screen that typically shows up after the screen has locked. After that, things seem to be normal.

What in the world is going on here? This has happened on all of our Minis, both recent and older, but happened on none of our new iMacs. It seems like very strange behavior... Any thoughts? Anybody else see this? Am I going about this upgrade all wrong?

p.s. I know --applicationpath is deprecated and generates a warning, but this was the way I sent the command originally and I wanted to be accurate.