probably relates to MC-88533

when summoning a marker armorstand at the edge of a block and trying to make it detect itself using dx/dz it will fail.

Steps to reporduce:

1. summon one armorstand at a fixed position:

/summon armor_stand 5 5 5 {Marker:1,NoGravity:1} 

2. try to make it detect itself using execute:

/execute @e[type=armor_stand] ~ ~ ~ /say @s[x=0,dx=10]

the command will fail. So will

@s[z=0,dz=10]

.

@s[y=0,dy=10]

on the other hand will succeed.

3. try the same thing with a fixed position not at the bottom of a block, like

5 5.5 5

-> it works now.

4. try the original coordinates with a non-marker armorstand
-> it works as well.