With an empty cache and hard refresh, there are definitely two different icons. (side note: These files have a 4-hour cache.)
The forum HTML has a relative link to the one in /forum:
<link rel="Shortcut Icon" href="favicon.ico" type="image/x-icon" />
Update the icon file, or just put a forward slash in front to use the site-wide icon:
<link rel="Shortcut Icon" href="/favicon.ico" type="image/x-icon" />
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