Outlook 2019 not syncing all Contacts in 365 Outlook web app. Syncing 100% in mobile devices/apps.

Creating a new email using the Outlook web app or one of my mobile devices' email app (connected to this 365 business account) perfectly offers any recipient name the user begins entering.

But using Outlook 2019 on my PC with the same 365 account has some but not all of my contacts. 

Under My Contacts, folders Contacts and Shared are present, Shared is empty.  Current view is People.

I have read a little on this, there are 6 old sync errors in the Sync Issues folder(Apr 2020). None newer.  I have verified the Firewall is open for Outlook. 

Sync Issues:

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8:04:17 Synchronizer Version 16.0.10350
8:04:17 Synchronizing Mailbox 'UserName'
8:04:17 Synchronizing Hierarchy
8:04:17 Error in folder 'Conversation History'
8:04:17 [80004005-501-A47-733]
8:04:17 The client operation failed.
8:04:17 Microsoft Exchange Information Store
8:04:17 For more information on this failure, click the URL below:
8:04:17 https://www.microsoft.com/support/prodredirect/outlook2000_us.asp?err=80004005-501-a47-733
8:04:17 3 folder(s) updated in offline store
8:04:17 Done

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It's ironic that its this much of a struggle to use Outlook by Microsoft with a Microsoft 365 exchange account, while other 3rd party apps work perfectly immediately, without the struggle part..

What would be a first step in correcting this?