I also found this regarding UK extended keyboard. (Perhaps it's just a setting in windows somewhere?)
Windows XP SP2 and later also offer a “United Kingdom Extended” keyboard layout mainly intended for Welsh-language input. This changes the grave accent key to a dead key and adds ẃ, ý and ç to the AltGr state (AltGr+W, AltGr+Y, AltGr+C respectively). In addition, AltGr+2 produces a dead umlaut ¨ (AltGr+2, O gives ö), AltGr+' a dead acute accent (AltGr+', E gives é), AltGr+# a dead tilde (AltGr+#, N gives ñ), and AltGr+6 a dead circumflex (AltGr+6, W gives ŵ).
I reported this long ago. Using a Danish keyboard, there are several AltGr combinations that cannot be directly typed into MM. @ for example, which is AltGr+2. Some work, some don't. It's pretty much hit or miss. One shouldn't have to know the ASCII codes to type what one wants. It really does need to be fixed.
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Now when you mention it. I have disabled all my hotkeys in MM to get around that problem. That's OK with me since I don't use any hotkeys but it certainly would be nice if it was fixed.