Ok i will try, but this is the steps for replicating the issue.
takeoff normally, executing takeoff flows and so
ap1 on, autothrottle on, thrust lever on CLB.
in cruise speed say, set speed to 300.
Fo answer, set airspeed 300, and sets the speed dial correctly to 300.
after 15 minutes or more time.
say set speed 250, FO answer, set airspeed 200, and starts to increment the speed dial forever.
after this i say: set altitude 10000 and FO answer set altitude 10000 and sets the selected altitude correctly to 10000.
all of this in selected speed mode and knots display. i never use managed speed.