command selector z and dz may not working properly

The command selector z and dz may not working properly. This is the command "kill @e[type=armor_stand,x=_2,y=~3,z=,dx=-6,dy=2,dz=0]". The command was not working like usual, as usual the command will just kill the armor stand at the top of quartz block, but now it will kill the armor stand at the top of prismarine block. （Screenshot attached）

Sorry for my bad english.