Post Exchange 2007/2013 Migration - Unable to Expand 1 Attached Shared Mailbox

Here's the scenario:

Exchange 2013, Windows 10 Pro, Outlook 2016, DFL/FFL 2008

Followed all articles I could find online as I went through my steps below before posting the question.

-Moved all mailboxes and public folders from Exchange 2007 to 2013
-Upgraded all users to Outlook 2016
-Mail-flow is working and users are not reporting any other problems
-Got 1 report from a user that had an attached shared mailbox that won't expand anymore
-Confirmed the user is still setup with Full Access (removed and re-added via EAC and Powershell)
-Applied Full Access to the shared mailbox during these tests (confirmed mailbox connects automatically)
-Tried for myself (Outlook 2016 Cached Mode), same results, but was able to add and view calendar but no meeting detail
-Tried with Administrator (Outlook 2010 Online Mode), same results
-Moved problem mailbox to a new test DB on 2013, same results
-Created new mailbox/user (domain user only membership) on 2013 and gave it full permissions to the shared mailbox
-Logged into test machine with this new user account and able to see and expand shared mailbox and view calendar detail
-Tried an old test user that was migrated from 2007 to 2013 and am able to expand the shared mailbox and view calendar detail
-Already tried a new mail profile (Cached/Online) for myself and Administrator same error when trying to expand
-Confirmed ability to open other shared mailboxes using auto-connect/manual methods
-Also confirmed same results for NotWorking shared mailbox whether using auto-connect or manual
-Tried my account on a fresh test machine in case it's a Windows profile issue, no luck

-Began focus on security settings for mailbox
-Ran Get-MailboxPermission -Identity 'execmailer' -User 'S-1-5-21-80640775-2141587