Hi,
Sorry I don't have manuals with me but I think it might be the flight crew training manual, or it might just be a company procedure.
The 2.5 percent is only required for second segment (as on take off) which usually ends before 1500 feet. If a higher gradient is required, there might be a higher MDA (DA) published on the approach chart if unable to make the required gradient.(eg Dalaman)or a higher gradient specified. The LPC calculates (F5) missed approach gradients which are usually well above 2.5. (not sure about A321). The chart may also specify a minimum height for level acceleration(eg Marseille) Or maybe a special procedure when single engine if unable to clear obstacles (Innsbruck)