Hi Michael,
I believe that your IT department is right.
It is probable that the problem in v1803 is associated with the "Periodic Scanning"
button. Please see my posts in this thread for August 27 and 28.
I think there is a fault in v1803, that leaves only some PCs without a functioning
periodic scanning button.
In other threads, some have had this same problem, and their button was turned on.
Yet, when they uninstalled their third party anti-virus, there problem was corrected.
A functioning periodic scanning button allows Windows Defender to work in concert
with third party anti-virus programs.
EDIT:
I did not have this link in hand when I first replied to your post.
This link is my case in point.
Andrew had the same problem, that was originally expressed in this thread.
He had also been successfully running a third party security program, in concert with
Windows Defender, until W10 v1803 was installed. And his periodic scanning button
was turned "ON".
Pay particular attention to the second page of his thread.
Regards, Glen