When I use the command — testfor @e[x=,y=,z=~,dx=0,dy=1,dz=0] — there is an extra 1 block distance detection in positive coordinates. Another example — testfor @e[x=-1,y=,z=~-1,dx=2,dy=1,dz=2] , it is a 3x3 area but because of the bug , it is 4x4.
Further explanation — base on the previous example , it should be only 3x3 , but because of the bug , it is +1 block on the positive coordinates which is +x and +z , it became 4x4.
Thank you for your report!
However, this issue is already being tracked in MCPE-174949, so I will be resolving this ticket as a duplicate.
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