People forget that they're _actively_ appearing to break it by using this UA detection. No company should _ever_ use user-agents for feature detection, because user agents themselves are screwy in so many ways [0]. Honestly, I feel like this was someone actively deciding to break the system on Linux (or any unrecognised UA). Wouldn't any reasonable developer just send back a fully-featured page if they couldn't understand a UA?
I have a feeling that this is the latter case. The detection code was likely written to give a higher QOS priority to direct browser access, and a lower QOS priority to the native OneDrive sync service built into Windows (because it happens in the background.) But in doing so, they probably made the check ask the wrong question—"is this a browser" instead of "is this our client"—and thus wrote the check with a browser whitelist pattern (that was never tested outside of UA strings from browsers on Windows), rather than a blacklist pattern (their one sync-service UA.)
Wrong.
There are plenty of bugs that cannot be detected using feature detection, especially with events and <input> elements. Sometimes you can find a browser-agnostic workaround, but sometimes the only solution is to sniff the browser.
It is especially necessary if you need to support "obsolete" browsers that your customers still use.
It is important to really try and avoid browser sniffing when "fixing" bugs in current versions of browsers (since your "fix" is likely to break in future versions).
Note, feature detection usually implies that you are using JavaScript.