Outlook Add/Delete Meeting Attendees Notification issue

I have a few Outlook 2016 Click to Run (v1803 b9126.2116) that are exhibiting a very weird behavior.  Unfortunately, the CEO and her very harried admin assistant are among those effected.

What happens is that when an attendee is added or deleted from an existing calendar meeting, and that meeting is closed, the user does not get the prompt asking whether to send the change to just the new/or deleted attendees or all attendees.  It simply sends an update to all attendees.

Outlook behaves correctly in Safe Mode.

• It does not behave correctly:
• In /safe:1 mode
• In /safe:3 mode
• In normal mode with 100% of plugins disabled and all ribbon customizations reset to default

I have rebuilt the profile because the OST is 40GB and the user is on a slower link and becomes very upset when this has to be done.

Anyone have any ideas what I can do to isolate and fix this?

Thanks in advance!
Jenny