Maybe I’m crazy? If you have an iPhone, enable faceid and put it into lockdown mode manually (I just remember it happening automatically).
Tell me how you get out of lockdown mode.
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.