I just ran into this same problem and think I solved it. It has to do with the URL that is saved with the USAA entry in the LastPass vault.
To solve it:
- Log out of USAA and close that browser window.
- In a new browser tab or window, enter usaa.com
- Click on log in, in the upper right corner
- Your URL should now be something like: https://www.usaa.com/inet/ent_logon/Logon
- In the USAA entry in the LastPass vault, paste that URL as the URL at the top of the entry and save.
- Now you should be able to use autofill, at least when starting from a clean new window.