the interface (iconnectAudio4+) was on - fader movement gets recorded
with the iPad on it's own there was in fact no such thing happening
made another test with an Alesis ioDock, which worked equally well, so probably any interface is ok

cheers, Tom

I'm sorry Tom, i don't follow...
My interface (Focusrite) has to be on while I am fading?

yes, the feature seems directly related to the presence of an audio interface
with just headphones plugged in, it doesn't react to the input fader

btw I've just been pointed to MiMiX, which maps Audiobus channels to physical outputs of a multichannel interface

cheers, Tom

FINALLY! By remaining connected to my interface, I was able to fade my song.

Thanks so much for your patience and help Tom!

And thanks for sharing about MiMix, that looks like a worthwhile investment.