Bathroom cladding is pretty rigid so there is little or no flexing once it is in place and being made from PVC it is impervious to water. If you want a totally rigid panel then the cladding can be fitted to sheet of thin plywood and installed as a one-piece panel.
We do not recommend installing our wall panels down behind baths - we would always suggest that you fit the bath first, making sure it is rock-solid and totally sealed, and then run the wall panels down onto the top surface. This will mean you will have off-cuts of cladding that can be used to make the bath panel and give your bath that "built-in" look so it is an economic use of the cladding as well.