Big problem, call the builder...............asap

If you know the waterproofing was not done properly why have you not already dealt with this , that is what your new home warranty is for

A lot of idiots are building leaking showers out there. And then worse idiots try to patch something without any proper waterproofing to begin with. Its all got to be ripped out and redone properly. With a complete waterproofing system, on walls and floors, like Schluter.