Microburst nailed it
Also be aware that the speed band the FM generates may be greater than ATC or company SOP allowed tolerances. For that reason if the speed decreases in the example you illustrated, you may need to manually select speed to force the speed back up to the target airspeed, even though it was within the target speed band. Not doing so is a sure way to fail your next check ride
Once back at target speed you'll be able to reselect managed descent speed. If repeated selected speed intervention is required, re-enter your destination temperature and a new (hopefully more appropriate) descent profile will be generated.
I'm not sure that was a very good explanation, so I trust that makes sense.