Two different user agent strings for one version of one browser? That can change at any moment based on a list the site authors don't control? There's no way that'd ever confuse things!
It's lazy/incompetent practice, which shouldn't really excusable for such a supposed massive advocate of web standards. MS are no shining example, but it's pot calling kettle black.
Definitely a Google bug - a long standing one
Some things aren't feature detectable, like actual weird bugs in behavior that need browser specific workarounds.
In this case it looks like MS has the domain listed for their fallback mode, which doesn't just change the UA but actually changes the browser behavior (IE9 in IE8 mode has no canvas support and will fail the feature detection for canvas appropriately).