Mac Mail default font Helvetica Neue Light reverting to Helvetica Neue Regular

Richard,

Thanks for the update. Next, create a new, test user account on your Mac to see if the issue remains there.

Add a user or group on Mac - Apple Support

"Add a user

1. On your Mac, choose Apple menu  > System Settings, then click Users & Groups  in the sidebar. (You may need to scroll down.)
2. Click the Add Account button below the list of users on the right (you may be asked to enter your password.)
3. Click the New Account pop-up menu, then choose a type of user.

• Administrator: An administrator can add and manage other users, install apps, and change settings. The new user you create when you first set up your Mac is an administrator. Your Mac can have multiple administrators. You can create new ones, and convert standard users to administrators. Don’t set up automatic login for an administrator. If you do, someone could simply restart your Mac and gain access with administrator privileges. To keep your Mac secure, don’t share administrator names and passwords.
• Standard: Standard users are set up by an administrator. Standard users can install apps and change their own settings, but can’t add other users or change other users’ settings.
• Sharing Only: Sharing-only users can access shared files remotely, but can’t log in to the computer or change settings. To give the user permission to access your shared files or screen, you may need to change options in File Sharing, Screen Sharing, or Remote Management settings. See Set up file sharing and Share the screen of another Mac.

1. For more information about the options for each type of user, click the Help button in the lower-left corner of the dialog.
2. Enter a full name for the new user. An account name is generated automatically. To use a different account name, enter it now—you can’t change it later.
3. Enter a password for the user, then enter it again to verify. Enter a password hint to help the user remember their password.
4. Click Create User.
5. Depending on the type of user you create, you can also do any of the following:

• For an administrator, select 'Allow user to administer this computer.'"

Take care.

Hi Richard Keleher,

Thanks for using Apple Support Communities. We understand that font preferences aren't saving in macOS Mail. If you haven't already, close and reopen the Mail app to test the issue again. Closing an app can resolve unexpected app behaviors.Quit apps on Mac – Apple Support (UK)

If the issue persists, restart your Mac. Restarting is a simple step that can reset connection and also resolve many unexpected behaviors.Shut down or restart your Mac - Apple Support

Let us know the results. Be sure to include any other steps that you may have tried.

Have a nice day!