Good lord, yes, you are right, "Bake Pivot" save my ass here. So basically my problem is my model's transformation was lost? And Maya 2018 rely on that to find out the symmetry? (btw without the transform info, even mirroring the mesh doesn't enable symmetry, which suggests the cause is lack of transform info)
Booleans are not breaking anything here. There could be multiple reasons why is this not working. First one you have to take into account is that result of a boolean operation is a separate and new object so if you have enabled symmetry on your original object only then obviously it will not work on the new object after…