Before putting the cases in the tumbler, I wash them in a bucket with hot soapy water. Drain them and then put them in the tumbler.
What are you adding to the water and stainless steel?
I also use this case lubricant. I have been using it for almost 30 years without any problem. It washes off easily with dishwasher soap and hot water.
You can buy it by the bottle or by the case.
https://www.dillonprecision.com/case-lube-8-oz-bottle_8_8_23666.html