I setup a completely new app registration in Azure and did everything all over from scratch, no change.
When I log into osTicket > Email > Email > Oaut2-Microsoft Config > and reentered the Secret ID and App ID, proceed to re-authorize, it brought me back to the agent login page once again. So I logged back in thinking something weird with caching is happening.
Repeat the above steps in osTicket and it brings me back to the Remote Mailbox tab I started off in. No "success" banner at the top and no Token tab in the Config area.
I'm trying to figure out what I'm missing here.