I see. So let me try and explain it verbally. Gonna redo the steps for generating this bug.

1: You are at Limsa adventurer guild, in front of guildleve corner (Upper Decks (7,5)). You have two entrances to that room...one north and one southwest.
2: You go to southwest entrance (that leads to southern part of upper decks map, the one where the aetheryte is). There you will find a small hall with wood texture at center and stone near its walls. Its in this hall where the bug is triggered.
3: You move to the center of the hall and then to the northern wall of the hall, as close as possible of the wall.
4: To confirm the bug you will need a second character. Once the first character is placed close to the wall in the hall (the hall is still at upper decks (7,5)) you target that one with the second character.
5: Now, wit the second character, walk back inside to the adventurer guild and then head to the north entrance (the one that leads to the stairs to lower decks and to the culinary guild) and take the stairs to the lower decks (still upper decks (7,5))
6: Going down that stairs you will reach a plain area with a long window that marks the zone between upper decks and lower decks. (upper decks (7,5)).
7: After the plain area you will see the second part of the stairs, that are already part of lower decks (7,5) and the character you placed at upper decks hall standing there too. If you kept that character targeted you will see that he will still be targeted, what means they do are the same character. You can repeat the path many times and the character at hall will still be appearing at stairs. Its a 90~100% chance of happening bug, which is good since its easy to reproduce. It may not happen in only 3 situations: a) the first character isn't close enough of the northern wall. b) the first character isn't around the middle (by middle i mean near the wall and half way out of the hall) of the hall. c) the first character is moving too fast (it can still be seen if the first charactter moves slowly).

I think its the most detailed i can be. I hope its enough this time...because more than that would be only making a video about it or showing it to a GM in person.