This is expected behavior. The so called `dirty` flag is not persisted over grid refresh, which is happening when you scroll above currently visible items in the grid (in case of virtual scrolling).
In the matter of fact there are few examples
available on how to preserve the `dirty` flag, but you should be aware that there is not universal approach to this.
Join us on our journey to create the world's most complete HTML 5 UI Framework - download Kendo UI