When I created a portal in the Overworld in a village and travelled to the nether, when I returned I spawned in a new portal on what I assume is the edge of the chunk. Upon destroying this portal I tried again and ended up in the exact same location in a new portal (pics below).

I then created another portal (note around 3 chunks away from the original) - went through, came back and I ended up in the portal above^ (pics for clarity) 

This is frustrating above all bugs I've ever seen on MC