Delegate on On-premises connected to Manager's Calendar with "Online" Problem

I have a dilemma that I can't seem to resolve. I hope that by posting here, someone will recognize the problem and help me fix it or at least point me in the right direction.

Delegate: On On-premises
Manager: On On-premises
Issue: The Delegate can create meeting invites and check the Manager's emails from her Outlook client without issues. However, the Manager is not able to see any new meetings after the Delegate creates them.

Research/Investigation:

1. The Manager experiences no problems on his mobile device because none of those are using cache mode. He can see the newly created meeting invites, etc. However, he cannot see them on his laptop and desktop.
2. The issue occurs when the Delegate's Outlook connects to Exchange as it should. The Manager's Outlook also connects to Exchange as it should. However, when the Delegate clicks/selects the Manager's mailbox/calendar items from within her Outlook client, it shows "Connecting to Online" instead of "Exchange".

Verification and Confirmation:

• All permissions are set correctly.
• Able to replicate the issue at other workstations.
• Followed all Microsoft suggested solutions.

The big question now is:

What causes the Delegate's Outlook client to connect to "Online" when selecting the Manager's mailbox/calendar items in her Outlook client when it should be connected to "Exchange"? And how do you fix that?