Hi @Hartwick, Anna , what's your Exchange server version? Did you enable the save contacts feature in Outlook for mobile?
Please try deny premissions for Outlook like contacts/ address book on your device.
You may also check if the question is caused by "Social Connectors" like this link introduces: Block Syncing of Personal Phone Contacts in GAL through Social Connector
If so, we will need to change the value in the registry.
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