When you summon armor_stand with the tag marker, you can't detect it in the negative direction with exact value using distance=x or with dx/dy/dz 

How to reproduce:

summon armor_stand ~ ~ ~ {CustomName:'"Center"',CustomNameVisible:1b,Marker:1b}

execute at @e[type=armor_stand,name=Center] run summon armor_stand ~1 ~ ~ {CustomName:'"East"',CustomNameVisible:1b,Marker:1b}
execute at @e[type=armor_stand,name=Center] run summon armor_stand ~-1 ~ ~ {CustomName:'"West"',CustomNameVisible:1b,Marker:1b}
execute at @e[type=armor_stand,name=Center] run summon armor_stand ~ ~ ~1 {CustomName:'"South"',CustomNameVisible:1b,Marker:1b}
execute at @e[type=armor_stand,name=Center] run summon armor_stand ~ ~ ~-1 {CustomName:'"North"',CustomNameVisible:1b,Marker:1b}

After that, if you do :

execute at @e[type=minecraft:armor_stand,name=Center] run say @e[distance=..1]
or
execute at @e[type=minecraft:armor_stand,name=Center] run say @e[distance=1]

Only the South and East get detected but not West or North!
however with:
execute at @e[type=minecraft:armor_stand,name=Center] run say @e[distance=..1.0000001]
Every armor_stand is detected!

The same thing happens with dx/dy/dz:
summon armor_stand ~ ~ ~ {CustomName:'"Marker"',CustomNameVisible:1b,Marker:1b}

execute at @e[name=Marker] run say @s[dx=0,dy=0,dz=0]

but no problems with:
execute as @e[name=Marker] at @s positioned ~-0.000000001 ~-0.000000001 ~-0.000000001 run say @s[dx=0,dy=0,dz=0]