To reproduce:
Add a RadGridView and use the following code to populate it:
for (int i = 0; i < 15; i++)
{
    this.grid.Columns.Add(string.Format("Column {0}", i));
}

for (int i = 0; i < this.grid.Columns.Count * 15; i++)
{
    this.grid.Rows.AddNew();
}t
Set the theme to TelerikMetroTouch.
Scroll somewhere below the middle leave the scrollbar and leave the grid with the mouse. You will notice that the value of the scrollbar will change.

Workaround:
Subscribe to the RadPropertyChanging event of the TableElement:
RadGridView1.TableElement.RadPropertyChanging += Property_Changing;

void TableElement_RadPropertyChanging(object sender, RadPropertyChangingEventArgs args)
{
    args.Cancel = args.Property == GridTableElement.RowHeightProperty ||
                    args.Property == GridTableElement.TableHeaderHeightProperty ||
                    args.Property == GridTableElement.GroupHeaderHeightProperty ||
                    args.Property == GridTableElement.FilterRowHeightProperty;
}