When you have had the disruption of builders I think it’s natural to be stressed. Personally I think the construction is correct. It’s just not finished yet and the post that slightly overhangs needs to be either chamfered at the bottom or an additional piece adding to make it look purposeful

I think you are right to have envisaged a stair newell post sitting completely on the landing. It looks like an error in measurement or manufacture to me. Your Architect will be the best person to help you come to an arrangement that is satifactory. Did you see the stair design before it was manufactured? Was there any prior discussion before manufacture of the exact finished width? It is possible that there was a communication error. Most important do you have anything in writing to confirm that the stair was going to be wider? Is there a stair width dimension on the Architect's drawing or was that detail left to the surveyor? You use the word "seem" which is an assumption.

If you look at pictures you can see it’s quite normal to have a post overhanging the landing