Can't automate user creation with preseeding
Solution 1:
Try using a hashed password like this:
$ mkpasswd -m sha-512
Password:
$6$ONf5M3F1u$bpljc9f1SPy1w4J2br5BWbE38lPlbBGgjbkJ6MnrvQK2kUfFyT/r0p.R22FFPMAc61B9BD/UWcXoui4qXInx01
And then in your preseed file:
# User account
d-i passwd/root-login boolean false
d-i passwd/user-fullname string My Full Name Here
d-i passwd/username string myusername
d-i passwd/user-password-crypted password $6$ONf5M3F1u$bpljc9f1SPy1w4J2br5BWbE38lPlbBGgjbkJ6MnrvQK2kUfFyT/r0p.R22FFPMAc61B9BD/UWcXoui4qXInx01
This works for me.
As a bonus, you don't have plain passwords in your preseed file anymore.