Here you can see the poor bond made between the Kerdi DS and the thin-set. When setting the membrane you need the mix of thin-set just right and you need to find the balance between applying to much pressure and to little pressure. If you press to hard you can run the risk of squeezing out all the thin set in an area.
Now by the book I would cut out the section. Mix up another batch of un-modified thin-set and apply some Kerdi DS, Kerdi or Kerdi band over the area ensuring a 2" overlap. But I like to double down on my waterproofing and was going to apply two coats of Ardex 8+9 that day anyway. So I simply used a little Ardex 8+9 to set the the patch and make the repair. I used Kerdi not Kerdi DS for the patch since it's thinner than Kerdi DS by a fair bit.
Here you can see the Kerdi patch on the left back primed with Ardex 8+9. I fill the flaps and work the 8+9 carefully from the center out and then from the outside in. Set the patch after I have applied 8+9 to both sides of the repair. I went on to coat out the entire shower pan and curb. and at days end about seven hours later applied a second coat of Ardex 8+9. Today we get to flood test these showers. So we can double check that the repair and the original installation is in fact water tight.
I'll show you the other tests as well - maybe tonight or tomorrow....