When the game tells you to complete Esto Yensa (the pyramid) and Osteinchyl in any order, if you complete Osteinchyl first and then visit Esto Yensa, an unknown game-breaking bug will prevent you from switching parties there (party-switching is necessary to travel through the dungeon).
I'm not sure if this is it or not, but I went back, and if I switched parties while NOT on a save marker, I was able to switch parties and complete the pyramid.
If it wasn't "being on the save marker and switching parties" that was causing it, then it has to do with party composition when you split up the teams. I forget the split when I was getting the crash, but the split when I didn't get the crash was:
Team 1: Vahn, Oalston, Siegfried, Nombasu
Team 2: Vera, Elina, Darius, Ami
No change in version - originally experienced the crash on 1.12, and didn't get the crash on 1.12 and got through the level, and now onto the merged world.