Error rate of capacitor between EXT_CAP and Vss

I'm using two AD5271 in pallarell, one for control brightness of LED and one for control sensitivity photo diode.
Datasheet discribes 1uF capacitor should be connected between EXT_CAP and Vss and the voltage rating of capacitor should be ≧7V.

My question is error rate of capacitance.
In my board, both AD5271 has 1uF capacitor between EXT_CAP and Vss, and most of boards works fine, but some boards fell into uncontrolable state of RDAC value, and the capacitor of these boards are corrupted in short-mode.

So I want to know the error rate of capacitance.
If you have any suggestions or information, let me know.