Screwing an metal MPT into a plastic fitting is NEVER a good idea or a "proper" procedure. The metal will ALWAYS corrode and create pressure on the plastic female thread. Tightening a hose clamp around the female fitting can minimize the possibility of it happening, but NEVER completely eliminate it. In your case, they may have used TPFE tape on the connection and it made the thread slippery enough that they overtightened creating stress which eventually cracked the fitting. The recommendation is to test the tub BEFORE installing it, for manufacturing defects, and also after installation to find any problems that could have occurred in the process of connecting the tub.