I have looked through the forum and documentation and have not come across any information on how Hubitat handles a lost connection to a critical device while armed.
There should be a communication heartbeat that HE is monitoring to make sure the device is still communicating and able to send a change of state back if once occurs. If a series of updates are missed, HE should assume that the device is no longer able to report its status back which should then result in the same action as if the "door opened" or whatever the device is.
In the Industrial Automation Safety world, this is done with normally closed circuits for hardwired devices and specifically requested packet intervals (RPIs) for Ethernet/IP devices that ensure the device is ready and able to tell you what it needs to tell you. Loss of communication is equal to the sensing of the unsafe action.
This all came about when my Smartthings multipurpose sensor that I am using on my garage door stopped communicating, luckily it thought the door was open and I did not arm my system for the night. What if it was the other way around? Would Hubitat let me arm my system with a non-communicating sensor?