And it is not required, but in some cases a good idea.
If you do not use thinset, then you do need to screw down the cementious board very well. Being upstairs and with the structure moving some, the thinset would probably crack anyway.
If it was my project, and being up stair, I would do both. In Texas, our soils are shifting so much it just would be a good precaution.
I would say the grout is cracking due to the shifting of the home. Just a slight shift or movement would cause this.
But then if he did not apply a galv, or zinc screw every six inches or so, it could be the bounce of the cement board.
Then to, the plywood subfloor could be the problem.
As you see, there are many variables.
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