I just spoke with LANDesk support and this currently a known bug with LANDesk 9.6 SP1 and SP2, though they're not sure why it's also happening in 9.5 SP3. I sent them a workaround that we've been using and wanted to share here too.
Instead of running the Scripted Install action, we have an Execute File action. The target is setup.exe file from the OS installation source folder. You will always want to point it to the x86 version of the OS install, even if you are installing X64 (at least for Windows 7. I'm not sure if this is needed for Windows 8 or 10).
The command line parameters are:
/noreboot /unattend:"path_to_unattend\unattend.xml"
Your unattend file should contain the path to the X64 install.wim in the ImageInstall section.