fidelity has a number of portals through which you can access their site. only the portal which comes up when you enter "fidelity.com" in the browser bar has the aforementioned problem with the fields.
try the following:
1. delete any saved passwords containing the domain fidelity.com from your vault (a clean slate).
2. go to the following url: https://login.fidelity.com/ftgw/Fas/Fid ... efaultPage
3. enter your username and password and login
4. save the site when prompted by lastpass
you will still run into a problem when typing "fidelity.com" in the address bar, but if you access the site by clicking on the bookmark in your vault (which will lead to the portal with the lengthy url above) and/or simply bookmark the longer url in your browser, the automatic login should work.