Hi mhmason,
It uses your profile's First and Last names' initials to populate the avatar as shown below. You can change your First and Last names from your profile, sign out, sign back in and the initials in the avatar will be updated.
https://help.webex.com/en-us/WBX24911/How-Do-I-Change-the-Host-Name-Displayed-in-a-Meeting
It seems like your profile's SSO is cached within the browser and has T as the First name's initial while A for the last name.
That is why when you logout and join the meeting from the browser, it uses your form's entries to populate the avatar at runtime.
Regards,
Hussain