There has been an OEB called 'incorrect vertical profile during non precision approaches', OEB 30 (A330) and OEB 42 (A320) in a effect for a number of years now.
This OEB explains that, due to a possible FMS coding problem, your operator has to have compiled a list with affected approaches, OR, you have to check the coding of the FMS against the plate yourself. Depending on the result you are either permitted to fly this non precision approach in Fully managed mode or laterally managed, vertically selected. So I agree with what vilas is saying.
I THINK the reason your company doesn't want you to ARM APP until you're established on the platform altitude is to guarantee that all the conditions for arming the FINAL APPROACH mode are met (this will automatically cause the blue arrow on your FAF to display). In other words to make sure that you're not going to be too high over the FAF.
If you would be too high over the FAF, I can see how thing can get a bit hairy quite quickly. On the other hand, if you arm APP early, you can check earlier if you have met all the conditions, but checking if the blue arrow is indeed displayed over your FAF.