In the Spitfire Solo Strings main patches, by default the Pizzicato articulation is mapped to switch to a Bartok Pizzicato within the highest dynamic range. 120-127.
You can tell that an articulation is mapped by the small 'white arrow' on the interface.
There are 2 ways to disable this:
1) "Alt" click the articulation to disable the keyswitch
2) "Ctrl/Cmd" click to change/disable the keyswitch.