That sounds like you have it on swell wet mode. To do what you want to do, switch it to swell dry in the menu and set the reverb mix to 100%
There's a ducking effect when in swell wet and swell dry modes. The swell only occurs when you let a chord or note ring out. Even if you play a staccato note, the swell will stop rising as soon as you mute the strings. The swell mode creates beautiful volume swells, but not really a pad sound.