It's just what happens with ALL these bath/shower mixer taps (rubbish)!
When you select the shower position on the taps a small portion of the water being mixed inside will inevitably find it's way back down the tap spout.
If you want to blame something you will have to blame the taps!
Just one of those things you'll have to put down to experience and next time fit a separate shower
http://www.screwfix.com/p/mira-coda-ev-2-thermostatic-bar-mixer-shower/77447?_requestid=304417