Using older versions of Outlook for Mac for personal email and calendar
It's a known issue and they are working to fix it. Your Exchange admin can view known issues in the service health area of the Dashboard.
E.g: https://admin.microsoft.com/#/servicehealth
They claim it's only happening for web, but it happens on Mac Outlook and iPhone as well.
If you look at older messages, you will see the icon is there, but on new messages that you reply to it will be missing. I honestly have not even noticed the icon before my coworker started to complain to me it was not there. Typically I remember the messages I reply to, so I have no use for this icon myself.
If it was up to me, we would be using Thunderbird, only problem is that it does not support Exchange out of the box. Microsoft need to support Thunderbird, possibly with a plugin, because Linux users are having a hard time checking e-mail. Never trust Microsoft, they will sabotage your software by removing features you rely on or ditching support entirely. E.g. Outlook Express, Windows Live Mail. Etc. The default e-maill client in Windows 10 is complete s-h-i-t compared to older Outlook Express and Windows Live Mail.
Edit: removed censorship of the word "s-h-i-t" by Microsoft, and also, the replied icon just started working for us again! Yay!