IE loses automatic UTF-8 encoding in iframe form target

Question

I have an odd problem in IE. It has to do with how IE detects the encoding of an iframe based on its parent content. My application wraps the content of a page in an iframe, and sets the encoding of the parent window to UTF-8 through the Content-Type header. The content of the iframe does not set the encoding through the Content-Type, and picks up the parent window's encoding on its initial load. This is the desired behavior - the content window requires the UTF-8 encoding for some language content, but for complicated reasons beyond my control, it cannot forcibly set its own encoding, so it relies on the parent window's encoding.

The problem arises when the content page is the target of a form action. When the form submits and the page loads in the content window, it auto-selects Western European (Windows) encoding. Does anyone know why? I've tried searching for any sort of documentation on related behavior, but the googles, they do nothing. Any sort of a lead (beyond sending a Content-Type header or a byte-order mark in the content) would be most helpful.

I unfortunately don't have a public place to host this, but copy-pasting these code samples to local files and saving each with UTF-8 encoding without a byte-order mark should consistently reproduce the behavior in all versions of IE.

frame1.html

<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<div>エンコード</div>
<iframe src="frame2.html"></iframe>

frame2.html

<form>
    <input value="エンコード">
    <input type="submit">
</form>

To recap with the example, if you load the page and check the encoding of both the parent and the iframe, you should see "Auto-Select" checked and "UTF-8" selected in both. If you hit Submit in the iframe, the frame will reload and the input text will be garbled. Checking the encoding of the iframe should still show "Auto-Select" checked, but now "Western European (Windows)" will be selected instead of "UTF-8". I need to know if there is anything else I can do to make it automatically preserve the UTF-8 encoding when the form action completes.

Thanks in advance!