Hello =) So. I have 4 portals.

Portal 1 is at : 8,y,8 in Overworld
Portal 2 is at : 56,y,56 in Overworld
Portal 3 is at : 1,y,1 in Nether
Portal 4 is at : 7,y,7 in Nether

By the "Nether portal coordinate conversion" rules :
Portal 1 coords should be divide by 8.
Portal 2 coords should be divide by 8.
Portal 3 coords should be multiplied by 8.
Portal 4 coords should be multiplied by 8.

So the intended behavior is :
Portal 1 should go to 1,y,1 in Nether (Portal 1 -> Portal 3)
Portal 2 should go to 7,y,7 in Nether. (Portal 2 -> Portal 4)
Portal 3 should go to 8,y,8 in Overworld. (Portal 3 -> Portal 1)
Portal 4 should go to 56,y,56 in Overworld. (Portal 4 -> Portal 2)

Result i had : 
Portal 2 teleported me at 1,y,1 in Nether. (Portal 3)
This break the conversion behavior.