Windows: Disconnect sometimes take a while for micro:bit to realise

[microbit-grace]

Repro steps

On Surface Book Windows computer:

1. Visit CreateAI on browser.
2. Connect to micro:bit.
3. Click "Disconnect" button.
4. Wait for micro:bit to detect it was disconnected and no longer show smiley face.

Sometimes it takes ~1min. Sometimes it takes 3.5 secs.

The impact on users mean that if the user click "Disconnect" and then "Connect" before the micro:bit has detected the disconnect, the reconnection would fail.

It seems to not be reproducible on the Minx mini PC.