When entering an over world nether portal, it is supposed to put you at a portal in the nether with coordinates roughly 1/8 of the overworld portal. It seems that randomly it will instead put you at a portal in the nether with roughly the same coordinates of the one in the overworld. Returning through this portal in the nether correctly multiplies the coordinates by 8, and now you are at an overworld nether portal with coordinates 8x the original portal. 

This has happened 2 times to players in my world. Here are the exact coordinates for one of those times:

The portal in the overworld was at x,y,z 6586, 67, 18162; This was linked to a portal in the nether at 823, 119, 2270.

When the bug occurred, the player entered the over world portal, and was teleported to a new portal in the nether at 6596, 84, 18155. Entering this portal to return to the overworld took the player to 52764, 70, 145247. 

This leaves the player in a position where they are forced to let themself die if they want to return home, therefore losing the entire contents of their inventory.

Absolutely game breaking. To any player that reads this before it gets fixed: I highly recommend carrying an enter chest with you when you enter a nether portal so you can save your items if this bug happens.