Hey Manu,
Thanks for your question and using our products

This is expected behaviour in Cuety. The app does not have a dedicated UV colour fader. The standard colour controls in Cuety are RGB, Amber and White.
For a UV channel in a custom personality, the best workaround is to map the UV channel to Custom 1 or Custom 2. This will give you a separate fader for UV, but it will appear as a generic custom control, not as a UV fader.
If your fixture does not use a real amber channel, you can also map UV as Amber. In that case the fader labelled “A” in Cuety will control the UV channel.
For an RGBWUV fixture I would normally map it like this:
Red = Red
Green = Green
Blue = Blue
White = White
UV = Custom 1
Hope this helps!