We're in the process of renovating a bathroom and things are pretty close to being done, but we've noticed that that water comes out of the shower head when the tub is being filled. The set up is pretty simple... It starts with a thermostatic shower body (all fixtures by Paini - www.paini.it). The water exits the top of the shower body and then goes down to a tub spout with a diverter. From that point it goes up to a shower head.

When the water is turned on it comes out properly from the tub spout, but after about 30 seconds water starts coming out of the shower head as well. There is a fair amount of water pressure in the building, but we've tried lowering the water pressure by adjusting the shower body and the problem continues.

If I understand things corrrectly gravity is supposed to keep this problem from happening. So what are the things our plumber should be looking for and correcting?