When you migrated your mail from an on-premise Exchange to Office 365, and you did not (yet) uninstall Exchange and/or clean up AD, Outlook will still try to configure itself to connect to the on-premise Exchange when adding a new account, even though Autodiscover has been configured correctly (autodiscover.domain.tld pointing to autodiscover.outlook.com).

The reason for this is in the way how Outlook performs its autodiscovery attempts. The first step in the discovery process is trying to configure the account using Service Connection Point (SCP), which is defined in AD (or in rare cases, in the registry of the computer). As the SCP is still configured in AD/registry, Outlook will use thos  ...