You did not give any details of exactly where the grout is cracking or the construction.
First of all it is possible that the plywood is pieced in or overhangs the last support just enough that it flex a little more than it should.
But a basic rule is that when ever tile buts up against a different material (such as a thereshold) or changes (such as floor to wall) grout should not be used.
The 2 parts will move separatetly for several reasons.
That space should be filled with caulk.
You can get caulk that is matched in color to the ground and sanded or unsanded.
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