Ok, you're probably a victim of WOW6432 redirection. As a 32-bit application, the BigFix client's accesses to HKLM\Software are redirected to HKLM\Software\Wow6432Node. See if HKLM\Software\Wow6432Node\Microsoft\Office\ClickToRun\Configuration has the CDNBaseUrl value you were expecting.
If that's the case, just add
action uses wow64 redirection false to your actionscript at any point before running the batch file.
You might also need to change the 'dos' command to a 'waithidden', i.e.
action uses wow64 redirection false
waithidden cmd.exe /c "C:\itsetup\Office 2016\Current Channel Upgrade\UpdateOffice.bat"