A disappearing lock is the a mesh issue and is probably not anything that they can solve for you.
However, I do have a suggestion based on what worked for me.. Install this driver which I ported from SmartThings and use it ONLY to configure the lock. Once it's working you can safely switch back to the stock driver.
Here's what do... With your lock connected, install this driver, change your FE599 to use it (Z-Wave Lock). Open up the debug logs in a seperate window, then click "Configure". You should see the lock communicating with the hub.
When the lock is finished communicating, try unlocking and locking the lock. If you do not see anything in the logs, then your lock is not communicating in the Z-Wave mesh. If you do, then try locking and unlocking, first from the Hubitat UI, and later from the locks keypad. You should see the lock reporting each event correctly, and instantly.
If you do, switch back to the native driver and leave it be. There's something in the ST lock driver that sets these locks up correctly to report local lock events. I know that the Hubitat team is working to identify it. In the meantime, see if this process will work.