don’t use flaps as speedbrakes, descent clean where possible
My guess is that, regardless of the manufacturer's recommendation, the vast majority of pilots aim to use flaps as speed brakes in their initial flap selection during descent, instead of planning to extend them in level flight or close to it. This may be due to their impatience to reduce speed while descending. Hard to do in some aircraft. There are always seemingly plausible excuses for the habit; one of which is that ATC might notice a momentary marked reduction in flight path angle as the aircraft is trying to slow up before initial flap extension?.