Wow – ok I do not read your language– but it looks like you set everything ok.
SO even when you change the Out of Stock to In Stock—- it changes back when you save the product?
You list a product, click manage stock, list the QTY as “1” , select “in stock” , and select Sold Individually. Click SAVE.
Now, it does not save your stock status as in stock? Is that what you are saying?
And if you list the stock QTY as 2… it does not show out of stock if you sell those 2?