Breakpoints and/or Release builds should not change the way a ListBox
behaves. So there is something else going on in your application.

Could you share a bit more information about your application? You mention
using Console.WriteLine calls. Are you still writing a Windows Forms
application? Is the listbox working okay interactively?

Note that if you manipulate components with a user interface (like ListBox)
in a piece of code, the UI might not be immediately updated unless you
implicitly or explicitly call Application.DoEvents. This might have bitten
you this time. Is the code you showed part of a larger code block? How and
when is the code executed?

Again, more information would be needed to solve the issue you are having.