You can test this on the WMF install by searching for "watch". Either "watc" or "watchl" will match for the project "MediaWiki-Watchlist" but "watch" does not.

From one of our resident JS gurus:

In T118539#1802886, @Krinkle wrote:

Probably because the JavaScript code in Phabricator treats an object as supporting arbitrary keys. watch is a non-standard reserved key in Object.prototype in Firefox's JavaScript engine.

The standard reserved keys are all in CamelCase such as toString, valueOf, hasOwnProperty. But Phabricator's script converts the value to lowercase before it searches, so those are fine.

Upstream bug! They need to use if hasOwnProperty.call( obj, key ) instead of if obj[key].