UnitWebRequest.isError is true when it shouldn't be

Unity 5.4.0

We're running into a situation where our server is returning a code 400 response (this is on purpose), yet very occasionally request.isError is true with the request.error message "Unknown Error". A normal (meaning positive) status code of 400 contradicts the fact that Unity is saying there was a low level error (i.e. negative status code). We get 400s all the time and they (correctly) do not have any relation to isError except when this bug occurs - about 1% of the time.

We used a third party HTTP library before switching to UnityWebRequest and not had this issue, so I don't believe it's a problem with our server.

Perhaps this is a bug with UnityWebRequests, since it's a brand new feature? Maybe fixed in 5.5? I saw in the 5.5 release notes...

• UnityWebRequest: Fixed case of exception being thrown when loading cached content. (822698)