Since your structure has symmetry in z, it has symmetric and anti-symmetric modes. Their bands are different. To have clean bands, the first step is to use symmetric or anti-symmetric BC in zmin to separate them;
THe second improvement is to greatly reduce the simulation: let the simulation terminates by simulation time, not by autoshutoff min. The autoshutoff level should be above 0.9, even 0.98. In such case, all simulation should terminates at 100%, but the autoshutoff level should be much higher than the original default value of 1e-5, as mentioned to be about 0.9~0.98.
If there is missing point, please check the corresponding file and check its autoshutoff level to see if it is proper.