yahoo mail login via IOS

I use ATT/YAHOO mail and have accessed it through all of my IOS devices for several years. Haven't had any issues until about a month ago. I could login to yahoo mail through the ATT webpage, no problem (proves I was entering the correct password). Every time I would attempt to login through IOS mail, I would get an "invalid password" message. I reset the password twice with the same result. No problems direct through ATT site but invalid via IOS mail.

I called ATT support, She gave me a temporary numeric password and instructed me to reset my password again (and create a new personal password). Miraculously, everything worked. Each device now logged in on first attempt. I don't understand why everything worked after the temporary password reset. I had done this twice. She claims she did nothing on her end.

Did the temporary numeric password possibly reset something other than my password?

This is more for enlightenment. I thought maybe a computer wiz could explain this. The tech could not explain much. Why it was malfunctioning or why it started working.

Not much to go on but comments are appreciated. Thank you!