I have tested this issue with the latest versions of Firefox (v80) and Edge (Chromium, v85).
When logging in to Khan Academy (KA) for the first time, the browsers offer to save the login credentials (as is expected). However, both the browsers only offer to save the password, and NOT the username/email; that field remains blank.
Even after manually entering (and saving) the username/email in the password manager, only the password field is autofilled the next time we log in to KA.
Is this by design? I would hope this is a bug, and that it would be fixed speedily.
Finally, thank you for this amazing resource.