ultrasoni sensor stuck to zero when motors are going backward

Hello, I have some troubles with the Robo:Bit Mk2.
I follow the instructions on your blog https://4tronix.co.uk/blog/?p=1534 
unfortunately the ultrasonic sensor is stuck to Zero when the motors are going backward (for example: drive speed -600) and stay at zero when motors are going foreward.
Resetting the micro:bit temporary solve the issue.