Question Microsoft 365 not syncing people & contacts ?

[MasterOblivious]

Edit: I tried sending an email to one of the problem addresses with the old Outlook and it had no issues. When I went back to the new version, the problem was still there
I'm trying to figure out why our Microsoft Exchange 365 isn't syncing the people tab with the contacts list.

When we enter in a customer's email address to the People tab, we give them a number so we can find their email address easily. So we would put it in as "100 First-name Last-name". That way when we only have to remember the number rather than the name.

Lately we've had this frustrating issue where certain people just don't come up when we type in their number. I've tried clicking on To to get the address that way but it's not there.

I have tried:

• Renaming the address cache
• Resetting the address cache
• Running and installing windows updates
• Restarting the computer
• Outlook.exe /CleanAutoCompleteCache
• Making sure the settings are turned on
• Exporting and importing contacts
• Making a new contact list for customers

The only things that changed was after I imported the contacts and make a new list. Outlook started eating up 70-80% of the memory after importing the contacts. As soon as I would hit save on making the new list, it would immediately disappear.

Our set up is as follows:

One computer uses the Microsoft 365 program
The other two use the Mail desktop app
All computers are logged into OneDrive and use Windows 10

If I need to add anything else, please let me know.