ActiveX is simply a container to run code in other applications. If there are display or data issues it's not the container's fault. I completely agree with the other folks, if ActiveX isn't working it's consistent on ALL platforms.
This sounds like a problem with the developers code. In fact it's a classic problem that sounds like there might be problems with the variable management in the control and that reinitializing the component. Subtle design flaws in code can remain hidden until the right conditions occur such as a specific server OS and even browser. However, that doesn't mean it's the fault of the browser and OS.
As an example, I'm actually fixing a web server component I wrote that streams files and it only happens on the x64 version of Windows 2008. I'm convinced it's my fault and not the OS. It is very intermittent and hard to consistently replicate, however, I'm sure it's my code.
Another example is last summer I had a bug that only happened on the x64 version of IE 7 on Vista. The problem didn't occur on the x64 version of IE 7 on XP. It was REALLY hard to find but ultimately it was a very subtle bug in my AJAX/JavaScript that was exposed only on that specific OS/browser combination.
I think you need to push your developer a little harder to dig deeper.
