The unreachable event is something @Kristopher put into the bulb devicetype I used as a starting point. It’s intended to monitor when the bulb doesn’t respond to a command, to determine if the bulb has been physically powered off at the switch.
It shouldn’t be able to physically turn the bulb on or off, only change the state in the app. Unreachable 0 means it got a response from the bulb. It will count up each time refresh() is called and at 2 it will change the state in the app to off.
Most likely it’s the poltergeists that ST has been experiencing lately that are turning them on. I wouldn’t mention the unreachable issue in the ticket because support sometimes can’t/won’t help with custom device types.