The problem is more complex. VM handles now the BE language different. So in my case it was so.
Shop default language in German (means joomla default FE language DE)
I installed the sample data and opened my product with the Backend language, english.
So I saw the "english" text of the sample data in the "english product", which was just the fallback of the "english text" in the german tables.
So when I stored the german text, the english texts showed the german entry and I got confused, but it was just the fallback.
Solution, use as default in BE also the default of the FE
and developer solution is to load the product with "getProduct" and "getProductSingle", and one time without language fallback, or so. The we could grey this fields, so that it is visible.
Normal derivated values are just not shown, but language fallbacks are shown always and that can lead to confusion.