Brand new VM with Home Assistant OS, KVM image on Linux. Everything unpacks and installs. At the Inital screen, “are you ready to awaken your home…” I enter my name and change the default username to my last name (not my first name). Enter a password, confirm, continue through the remainder of the onboarding process. All appears normal.
However, if I log out and try to log back in, with the username I typed, I get “invalid username or password”. By trial and error, I tried my first name and it logged me in.
The onboarding process seems to IGNORE any changes to the Username field.
I made a screen capture, mostly as a check to my sanity. Is there something I’m missing? Is this some strange undocumented feature?
I went to file an issue in GitHub, but frankly, I’m not sure which part of Home Assistant this belongs to. The core?
Well hallelujah!
I’ve just experienced the same issue with my first ever install of HA and thanks to your post, I may well be able to proceed 
Fresh install with 9.4 on RPi seemed to go OK until the onboarding process…
I entered my name which in turn caused the username field to become populated with the very same so I changed this to something else, entered a password and continued up until the analytics permissions 
. Despite checking and unchecking any, all and none of the permission options I just could not proceed from this point.
Unable to proceed and a little unwilling to attempt to work towards reinitiating the onboarding process only to have the same thing happen I decided to restart HA. Alas this was to no avail leaving me where I was just a few minutes ago being informed that my log in credentials were in someway invalid 
After a search here and a search there I stumbled into your post and thought surely not, an issue highlighted very nearly a whole year back couldn’t be my issue…or could it; by this stage anything is worth a go right??
Sure enough, I entered ny name rather than the username I entered and low and be hold I am in. Not a great start to my Home Assistant journey but I’m in now, I just hope this post helps others before they start thinking they’ve somehow lost their hand to eye to brain recognition…