the IsConnected property never was able to detect, that a connection is gone
see also Change Text when Bluetooth disconnected - #8 by Taifun
as the bluetooth protocol was designed, there is no way to detect, if a connection still is alive, therefore you have to send regularly (i.e. poll) some data to the device. And if you get an error, then you know, that the connection was lost…
see also A simple Bluetooth Chat with App Inventor 2
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.