Compass seems to evalute everything with JS, so if you enter {um:-2583493467308876869} it will treat the number as -2583493467308877000 and of course won't find what I'm looking for.

So the fix is to use {um:NumberLong('-2583493467308876869')}, which is a bit painful to type, and even if I do that, the Query History still saves {um: -2583493467308877000}, so basically it's broken ...