The water is getting out the weep holes not in under the slab.
It looks like and excessive amount of silicone was used connecting this drain and most likely you have only 3 of the many weep holes working. Weep holes need to be protected from blockage while installing the mortar bed and finished tile. I use Candle Wick and Noble Company's positive weep hole protectors for this step. There are many ways to do this this but sadly now available to you at this time.
What type of shower liner was used?
Did you document the process?
Was the installer a NTCA member or a "Handy Andy"?