I have a very similar problem when a dynamically generated iframe immediately gets stuck in the first src after loading, and even if the parent page changes the iframe src attribute every time it reloads, the iframe still loads the first.
After dealing with this for several hours, I realized that it was just <IE8 update error, and if the page navigates through a link or through a URL to the address bar, everything works as expected. Therefore, unless you refresh the page or refresh it by entering the address again, you should be fine.
Update:
The problem seems to be fixed if you set the SRC iframe attribute again, for example
iframe.src = iframe.src;
This seems to be IE8 to really respect the given URL.
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