I am also getting the same issue! Have to manually click on the Sales Tools every time to get it to populate the Bcc field on both my mac and pc, both using Office 365.
To be clear, I only experience this issue when using the Office365 plugin, as I have tested installing and removing completely the Outlook desktop version add-in from my windows laptop, and it then does not include the Bcc field automatically.
***The solution I have found is to pin the HubSpot Sales tool sidebar so that it pops up in every message. This way it automatically populates the Bcc field.*** This is a band-aid but this works perfectly fine for now.