Insufficient slope of the sewage pipe

I have illustrated it here in a layman's way.

1. The community channel at the property boundary was already in place.

2. The wastewater pipe and the inspection shaft were installed in August. By the builder.

3. The wastewater pipe that was led out of the base slab. Factory-made.

The slope between 1 and 3 was only 6 cm over a distance of 15 meters.

Now the general contractor says that the shaft (2) as a reference point was not present when the base slab (3) was poured and therefore the too deep wastewater connection does not constitute a defect.

So what now? Did the line and the shaft come from the client side or from the subcontractor of the general contractor? I'm slowly losing track... You do know what client side means, right?

I think I should start over again...
Bauseits means "does not come from us, but from the [GU]"

Was excavated again underneath the base slab afterwards and the pipe laid differently? Or how should I imagine the rework? Does it go down again in the shaft? So could one simply have taken another shaft or does it go more or less straight through there? What does the pipeline planning of the GU say?

As far as I understand, a lintel was installed behind the frost apron. The frost apron was chiselled open and the pipe behind it was laid upwards or the lintel was shortened. Unfortunately, I do not currently have a pipeline plan.

Unfortunately, I currently do not have any circuit planning

But was there one?