You stay signed in on a device while you keep using it. You're signed out when:
you haven't used Nexframe Resume on that device for 30 days
you changed your password on another device
you reset your password (this signs you out everywhere)
you changed your email on another device
someone chose Sign out other devices on your account
you chose Log out
If you didn't do any of these and keep getting signed out, change your password and choose Sign out other devices in Settings → Account.
Before you log out
If some of your work hasn't reached your account yet, usually because you're offline, we warn you first. Choose Stay signed in and it saves automatically once you're back online. Log out anyway deletes that work from the device.