Did the stair installer mess up and then the post and railing guy had no choice?

What did the drawings show? Can you tale pictures from above?

I would not be happy with that. I do not know much about stair and rail building but that seems wrong.

@millworkman Go to the link in OPs post. There are more pictures.

Ah thanks @vinmarks, missed that.

Looks to me that newel post should have been installed further onto the landing from what see.

I wouldn’t accept that. So much of it is wonky. In addition to the strange landing treatment, the spot where the stairs wrap around the corner is crazy, the rail should have gone straight into the post closer to the correct hand height for someone on the stair below the triangular one, then they rest their hand on the top of the post as they navigate the turning stairs, and rail comes out from post at correct height to continue up the stairs…
I have double posts very close together on my landing, that could have been an option.
Some of what they did could be due to code where the house is, but since they let you have the tight corner turning stairs rather than another landing, they are probably lenient in your area.

Yeah, I did not even pay any attention to the winder treads, Where are you located that that is even allowed?

For safety and consistency, I would recommend making that lower turn a landing instead of winder treads. It's odd that you have a landing on the upper area where the stairs turn and then it becomes winder treads on the lower area. This inconsistency introduced a high risk of human error when navigating the stairs, especially if someone is not looking, has their hands full, or is groggy. This is a big safety issue in my opinion. Landings are always better, but mixing a landing with winding treads is a recipe for injury.

Handrail requirements are part of the reason I rarely use winder stairs. All my treads and landings are rectangles.

The winders may be code compliant; you'll need measurements per this picture to verify:

If you aren't compliant here, you're getting a red tag which will make the rail problem look like a walk in the park.

I'm not seeing why blocking the post out at the riser wouldn't have been a better trade off than the jogged rail; certainly less conspicuous. I'm not buying the inability to secure argument of the builder.

And speaking of conspicuous, wait until the apparently flat sawn custom made jog piece takes a stain compared to the apparently quarter sawn main rail. You better have a hell of a finisher.

"What did the drawings show?"

I've never seen a set of prints that had specific rise and run included. The architect/designer gives the stair builder a guesstimate at finished floor heights and hopefully enough space between walls to shoehorn everything in while still meeting code.

Not that this is always a design error. Someone may want a bigger bathroom or two and steal the space from the stairwell. Can't do that without asking.