Can't delete admin user


I can’t remove the admin user nor edit its name, the solution provided here doesn’t work on nextcloud 12.0.4 (from nextcloudpi). I can’t delete the admin account (the 3 dots on the right don’t appear). I even can’t remove its admin privilege (the box remain ticked when I click on it in the Group column).

Is this a bug, is this expected or is there a solution to my problem ?
I thinks it’s a problem to have user named admin with admin rights as bots usually try the “admin” username among others like “root” and then try to brute force the password.