It is not really possible to implement it. At least not effectively. The thing is that without an hardware support (it does not have hardware support for such feature) it is not possible to send any pattern into the bracelet when the bluetooth disconnect. The only way how to do this would be setting an alarm into the bracelet for upcoming minute +1 and replace this alarm every minute - thus when phone would not be on reach and alarm not set, the bracelet would start vibrating (after this one minute). The thing is - this would require waking up the phone from deep sleep every 1 minute, which - as you can imagine - would drain the phone battery extremely quickly (in few hours).
So because of the reasons above we decided not to include such feature in our application, although we do have it implemented for several months already - simply for the fact that it would drain battery extremely quickly and it would cause people giving us bad rating on Google Play. We would have to wait for Xiaomi support for this feature, we really think they could implement it into the firmware sometimes.