Been struggling with this one a few days...
Central attempts connection (connUP) with peripheral.
Central gets connUP callback.
Sometimes, instead of bond, central gets connDOWN callback.
Central attempts again but gets connDOWN again, a few more times.
Central finally gets bond, and sends write-request,
but does not receive expected notification from peripheral.
Central turns on scan.
But gets no ad receive callback.
Can't do anything to restore scan.
Must reset to get it working again.