Toast++
Nexus is literally the worst.
It's not the egg that holds the info, the parents are checked when the egg is generated. It goes through a few PIDs to see if any will make it shiny for the ID/SID of the current game. If it will, it keeps it. When you trade it, the ID and SID are different and so you will revert back to the original 1/8192 chance since shininess is determined by the ID/SID and PID at the time of hatching. Hope that makes sense for you.