What were the visible indicators that the phone had entered lockdown mode? It usually requires user consent, followed by reboot.
iOS Lockdown Mode does not usually constrain the method of unlocking the phone.
iOS Lockdown Mode does not usually constrain the method of unlocking the phone.
Tell me how you get out of lockdown mode.
The behavior being described sounds a bit like malware. If it happens again, the best option is to Force Restart (VolUp, VolDown, hold side button until the device reboots), which cannot be intercepted by any apps which might be trying to simulate iOS system prompts.