Over the last few weeks more and more users are getting a random string of characters as usernames, even if they specified a normal one during sign up. No one on the community website is getting any info from Spotify as to what is going on.
During a chat with support I was told:
Updated on 2018-06-27
Thanks for coming to the Community!
As we look to improve the overall Spotify experience, we’re now automatically generating usernames, hence the random numbers and letters. You don’t need to remember this to log in. You can log in with just your password and the email address associated with your account.
During sign up, you can personalize your account by creating a profile name that’s displayed in the app. There’s no limitations to a profile name, so you can choose any you like :)
It’s not possible to log in with a profile name however.
Rest assured you don't need to give your generated username out to friends for them to find you.
There's info on sharing your profile here.