How was the tile set? Old tile removed? New shower floor pan? What else done for prep before new tiles? Any in progress pics?

I doubt the "tile" near the drain became soft. The softness is due to your setting material and/or underlayment failing.

I agree. The contractor used this mortar. Is it an incorrect product for a shower floor?

The entire old shower was gutted to studs and wood floor. A new waterproof membrane was installed under the mortar.