The default Array.sort stability is inconsistent across targets _because_ guaranteeing it would prevent reusing implementations already available in the targets. And differently than what you claimed, many languages (and most Haxe targets) don't guarantee sort stability in their default "sort" APIs: C, C++, C#, JS, PHP and Neko.
Also, any remark placed in the official api docs for "Array" or in that very module, just next to sort method, can't be said to be "buried" in the docs.
I'm not dismissing the existence of a bug just because of this (although the issue may be related to it), but non-deterministic comparison functions are problematic with many sort _algorithms_. You might experience (other) problems when using implementation independent "sort" APIs, since many algorithms need to rerun the comparison function several times and expect consistent behaviour; I would expect certain algorithms to, for instance, never complete.
I really don't mean to be condescending, but are you interpreting "stable" to mean "crash free"?