Get your money back, awful, dangerous stuff, and shouldn't be on sale to the General Public in my view. It's no good for cleaning drains and too many Dickheads ready to throw it in the face of the next person they take a dislike to.
Still say the problem lies with pressure issues in the pipework, as you flush the WC, if the pipework is sealed with an AAV at the top, and there is an issue downstream, the water flushed needs to displace the same volume of air. If it cant vent naturally, then it's looking for the next easiest way out, in this case possibly the shower. Issue is you've a more convoluted soil pipe setup than usual, can you get access to the section within the building to inspect it?