The game doesn't remember where you logged off and puts you in a random position somewhere in your world. The game saves constantly except for this. My realm partner went to the Nether shortly after updating his game and was spawned somewhere random in the Nether without a portal anywhere in sight. The portal he used was one which we had for months and constantly used and when I used it, it had no problems and spawned me in the right place. I logged off at home base and when I returned, I was in a random location.