..looks like it is a bug. I am facing the same problem. I have a combo chart where there are 3 expressions. (Current - shown with symbol and Max/Min shown using stock) and a single dimesion.
What I observed is if I have >100 values on x-axis, then it fails to show the Current as symbols ..it shows it using the line and when I have <= 100 values on x -axis then it displays the Current values with the symbol.
Can anyone please confirm if it is a bug?