I don't know if there is another solution to your problem but I see your wall reference lines are on the inside of the walls.
Technically a wall is only as long as its reference line so there is really no wall in the corner.
Archicad automatically fills in the gap so you see a true corner but it seems that this is not being translated in your IFC model (I don't know if there is an option to fix this in the translator maybe?).
Turn "Clean Wall & Beam Intersections" off in Archicad and you will see the same thing.
However if you move the reference lines to the outside of the walls I think you will find it will look better in the IFC model.
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