Regarding your question: “Is it always a problem with older, previously created worlds?”, this is the only world I've imported from a previous version, so I don't know if it's a recurring problem.
as you can see in the picture, i don't have the same structure than you despite the fact that i'm at the same coordinates
Seed : 218451881
Coordinates : 786, (y?), 834
I put the wordl in creative mode to find another strobghold but i can't. It always send me to this coordonates.