Cylinder head repair

hey guys !
I had fuel in my coolant system and had to replace to cylinder head and coolant tank. It coast me a lot and they also flushed the system with special cleaner many times.
I drove it about 1500 mills , I had diesel in it again. Flushed it again. Drove about 3 days I still have some diesel in my coolant.
Is this normal ? Or it still mixes the diesel and coolant ?

 

I can return.
But when I contact them they say it's normal cuz there is still diesel in the carburetor and it's hard to clean. I'm not sure if the problem is solved.

 

Diesel in my coolant tank

 

Cummins isx.
2010