It is for sure a bad shower wall install. First, the shower walls are not wide enough - need to be as big as the shower base. Those shower walls are notorious for causing water problems - they are impossible to seal effectively. I'd pull off all the plastic shower walls, remove the drywall and replace with GoBoard or Hardibacker. You can put a drywall corner bead on after you repair that corner. Order your new shower kit (the correct size this time) and install per manufacturer instructions. Make sure you use a high quality silicone caulk at all corners and at the shower base.