Example

execute @a ~~~ detect ~~~1 portal 1 setblock ~~~1 air

And 

execute @a ~~~ detect ~~~-1 portal 1 setblock ~~~-1 air

work fine(command and command block)

but when you do

execute @a ~~~ detect ₁~ portal 1 setblock ₁~ air

and

execute @a ~~~ detect _-1~ portal 1 setblock _-1~ air

it fail(command and command block)

Portal facing x coordinate have a problem I guess