TouchOSC fader dropping desk fader to BO?

Hello forum,
If I adjust a virtual fader on iOS/TouchOSC/Hog4 template I see the change on RoadHog 4 - but removing finger results in BO on console master. Why is this; a limitation of OSC/non motorised faders or something wrong with the settings?

The manual's a bit thin on these details. Any help gratefully received.
Thanks. D