Folks, we are having the same issue at my site. Users are logging in with their own credentials and using the Outlook plugin. If they send an email to a contact already in HubSpot, they immediately inherit the customer and contact and take it away from the original owner. This is wreaing havoc on our process.
No settings were changed - so they are on default. Yes, it happens immediately after, and we need to change contact owner manually. So far it happened only for one of the team members, but with all of the emails sent.