they look like an empty ComboBox until I manually refresh
This is actually a bug in the layout (as strange as that might sound).
Feel free to add ticket...
As a workaround, until we get that particular issue fixed in 5.2, it should work if you add C and D when they are needed (instead of adding in advance and unhiding). That is: don't add hidden NativeSelects, only visible ones. This is not an optimal solution, I know, but should work.