This is not settling. This is bad carpentry. The gaps are PRECISELY the same on each and every tread. No house 'settles' that precisely. And if it did, you have more problems with the house than the badly cut stringer. It's the stringer not the house. Which is VERY good news.

am I the only one who thinks these gaps are small enough for caulk? mask first so you don't get it on the wood treads, and paint over it when done. you'll have to pull up the masking tape before the caulk dries, and then mask again for paint. don't cheap out, and look for a caulk with good stretch. I heard good things about Big Stretch: https://www.lowes.com/pd/Big-Stretch-10-5-fl-oz-White-Paintable-Latex-Caulk/4411173

OMG their workmanship is terrible. Obviously you didn't have a permit, because no City would have approved this job. You need to cut your losses and have this redone by a professional tile person. You should NOT under any circumstances be giving them advise on how to fix this. You will be giving them an out, because all they need to say is "She told us how to do this and we were just following what SHE wanted. SHE knew we weren't tile professionals. What you are seeing is just the tip of the ice berg of problems. I agree that probably the drainage of the floor is probably also wrong. Did they also do the Plumbing as well? In the future, you will undoubtedly be faced with major water damage to your home. If any damages occur down the road, it may be difficult to get your insurance company to pay for repairs once they see how this was done. These people need to return your money and the cost of your tile so you can redo the shower. For goodness sake don't let them work on any thing else in your house. If you need references, I would go to a large reputable tile store and ask them for people they know and trust. Be sure to check that their license. You can do this by checking their license no. with your State's Contractor's Board. Best of Luck to you.

>I fear the next post will be from somebody who can diagnose the level of waterproofing from >your middle photo...) >What waterproofing? You can't see that telltale red line in the side view photo? :-) OP: Consider putting tiles around the outside, in a border around the doorway. They look like they have a clean edge, so if the thickness matches the amount that the tile sticks out past the doorway, then that would work. If the tiles only stick out on one side, well, then the tiler needs to come back and do a redo. Remove and recut the tiles. I would tile the ceiling in the shower.