The stair steps are all different heights

Thank you very much for your answers

I found in the documents that the staircase was planned with 18.4 / 29.8. I assume this means that the height of the steps should be 18.4 when finished.
This would result in me having to compensate 6 cm height for the lower step according to the plan. That can't be correct...

Definitely a defect

We also had significant height differences. This is quite normal with constructed stairs (regardless of defects or not). The screed layer applies quickly to fit - that's how I know it, unless the tiler works with a lot of adhesive.

This is not good work. Ask how it is supposed to be finished - if it is to remain like this, it is a defect.

I wouldn't even know where to start correcting the heights further forward. Every step is affected, and not evenly. It would be easier with a slide without steps

Cast-in-place concrete stairs always vary slightly in height. They are rarely ready for finishing.

The "first step" with 12 cm is not really a step, as the floor structure is still missing. The other steps vary within an interval of 3 cm, which the tiler will even out. The 24 cm at the top would need to be looked at again to see exactly what happened there.