Z-Wave device disappears

I have a couple of Boundary PIR sensors. Set one of them up to control automations, just turning some lights on. Great stuff. However… every so often this sensor stops working and disappears from home assistant entirely. The device is just gone. It’s not missing or asleep, it’s entirely disappeared. If I reset it I can add it again and of course I have to add the device to the automations again.

There’s no apparent pattern. It can work for a few weeks or a few days before it disappears again. Notably the device itself changes behaviour too. No blue light on motion - and it isn’t possible to tell it to exclude, it behaves as if it’s totally reset.

Why might this happen?
If the sensor is dodgy I would understand it going offline but not disappearing entirely.

The zwave controller is an aeotec gen5. There are 15 other zwave devices that all work fine.

I’ve added the second sensor to see if this behaves the same way… no issues so far but will need more time to be confident.