John Doyle
Ansys Employee

From the images you shared, your mesh looks a bit coarse.  I can reproduce similar result with a simple block model under acceleration load, constrained at six corners and with a coarse enough free mesh.  The fix in my test case is to refine the mesh and make mesh perfectly symetrical about the plane of symmetry of the loading.   

See screenshots below.  


Topo-Opt result with coarse free, nonsymmetric tet mesh:


Topo result with finer hex mesh, perfectly symmetric:.