In the previous release of the Javascript Blocks editor, adding a block to inititiate the Bluetooth Magnetmeter service, would prompt you to run the calibration routine on the micro:bit

It is now the case tha the bluetooth magnetometer block won't trigger the calibration on it's own, so it has to be initiated via BLE eg from an app, or by using the calibrate compass block in your program.

  



 The calibration is stored in non volatile memory, so you won't need to calibrate every time you turn on the device - just the first time after programming.