When using negative dx & dz values as an entity specifier, the "detection square" has a weird offset from where you execute it;

When using dx=10, dz=10 it will detect you with in a square which is 10x10. (Like it should)

https://imgur.com/wcWWZp4.png

When using dx=-10, dz=-10 it will still detect you within the square, but the size of the square becomes 12x12, and the square will have an offset from the entity you execute it from

https://imgur.com/FVf7gYL.png 

Here is a small video showing the bug in action;

https://youtu.be/UecqpJ1wH-0

(Command used in gif:

/execute as @e[type=armor_stand] at @e[type=armor_stand] run say hi @a[dx=10,dz=10]

/execute as @e[type=armor_stand] at @e[type=armor_stand] run say hi @a[dx=-10,dz=-10]

)