As far as I can tell, there isn't a Microsoft product called "Outlook 365". So I assume you have an Office 365 account and that you are using the web email account through the URL
https://www.outlook.com. Is this correct? Or are you using the Outlook email client installed on your PC or mobile device?
Exactly where do you see your Fastmail email address with an adjacent (1)? A web login to Outlook normally accesses your outlook.com/live.com/hotmail.com email accounts. Do you have your Outlook account set up to access your Fastmail account? Or have you sent to or from your fastmail.fm account using Outlook?
In some cases Fastmail adds a number in parenthesis such as (1) (2) etc. to the subject of a message if you appear to have an email loop. This is when an email is sent from your Fastmail account in response to a previous email from your account which is looped back to your account. This can happen, for example, if you have a notification message which is delivered or forwarded back to your account, generating another notification message. The (1) (2) etc. addition to the subject prevents the loop from repeating past (5), I believe.
Bill