painfully, I had to figure out that the cheap chinese stuff I bought is just cheap chinese sh*t.
I bought these https://www.ebay.fr/itm/273559623184. One of them hat a little rusty spot, so I cleaned them all in 15 % phosphoric acid and gave them a bath in citric acid later, to cure the ss surface. They all looked pretty clean, indeed the bolt was made from a different material I guessed. Bit darker in color.
Today, they look like this

The body looks ok, but the bolt seems to be the problem.
Any advice what stainless steel needle valve to buy?
Thanks in advance.