When you deploy a system from a disk image (i.e. without running the traditional installer), an initial setup utility will run on first boot. For Workstation and Silverblue images this is gnome-initial-setup; in all other cases it is the separate initial-setup application.
This application should require you to create a user account with administrator privileges, or set the root password, before proceeding. gnome-initial-setup fulfills this requirement by always creating an admin user. However, initial-setup will allow you to create a regular (non-admin) user and then exit. If you do so, you will have no easy way to administer the installed system.
This bug is not technically specific to ARM architectures, but it is most likely to be encountered there as disk images are most commonly used for deployment on ARM systems.
Insufficient internal logic in the initial-setup.
The best way to avoid this problem is, of course, to create an administrator account or set the root password before exiting initial-setup.
If you inadvertently do not do so, you have a couple of options. You could just re-deploy the system and, this time, create an admin user or set the root password.
You can also boot with the kernel parameter
systemd.debug-shell=1; when you do this, after the system boots, you can hit
Ctrl+Alt+F9 to access a root console. Here you can run
passwd to set a password on the root account and reboot.