There is a certain VOR approach in our route network whereby if we arm the APP prior to reaching the FAF platform altitude of 2600ft, it does strange things. We were descending to 2600 and the "brick" constantly shows that we are above the profile (full deflection down) when we are certain that we are not and we will be able to be at 2600ft well before the FAF. And then prior to the FAF the brick comes up and the aircraft intercepts the vertical profile, leaving us hanging at about 3000ft below intercepting the final descent slope. So we are encouraged to arm the APP only at the FAF platform altitude.
I still don't quite understand what exactly the FCTOM means by "stabilised laterally and vertically before the point of descent starts". Because in this case we will definitely be level at 2600 at the FAF, let alone be within 150 feet and yet this happens.