A very unusual behaviour

Another explanation is timing... there are, I'm assuming, events leading up to your actions on the combo box, perhaps opening it's screen.  If the test scripts are running faster than the application under test, it's possible that the actions are attempted before the box is fully enabled and ready to be interacted with.  I've seen this happen many times in my own applications under test.  Your best bet (and this is a good best practice for any application under test) is that, especially if you're opening a new form, web page, panel, etc, in your AUT, add some sort of method or code to wait for that form to be fully ready and available for interaction before you attempt to interact.  With web applications, this is usually by calling the "Wait" method on the page.  In a "past life" when I was testing a Delphi application, I wrote a custom routine that I would call every time I opened a new form to make sure it was active before I started any interactions with it.  Any number of techniques can be applied to do this.  But the behavior you've described is almost certainly related to the timing.