Let’s talk about the unsexy part of tiling…
Thinset is essentially the glue/motar that holds tiles to the floor, walls, and even ceiling.
When demoing this bathroom you can see the left over bits of it and how ineffective it was in bonding to the substrate. You should be at a MINIMUM shooting for 90% coverage. Those sections where you can see the left over trowel marks should have been everywhere.
When tiling you need to test for coverage throughout to make sure that the trowel size, setting time, and the proportions all match up. I used this makita demolition hammer (Think baby jack hammer) when all I needed was a big screwdriver to get it up…
Now there are a couple things in the photo that were done incorrectly besides testing for coverage. First is the hardy backer on the floor, I will say that it is better than going right on top of plywood but there are way better membranes and underlayment's we can use. If you are going to use hardy board (Think thined out concrete board)
Beyond that the previous layer of laminent flooring was never removed. So now we have several different rates of expansion and contraction going on than just two.
Next time there is tile being laid?
Make sure it is a clean slate, use the proper underlayment, and check for coverage.