Why Does JQL Only Sometimes Return an Error when Issue Does Not Exist?

[Jacob Reeves]

We have noticed the following behavior when making JQL based query in either advanced search or the API. 

Given an issue number that does not exist and a query: `key in ('QSTNG-3982')`

* Running the query with no other keys returns an error: 'An issue with key 'QSTNG-3982'' does not exist for field 'issueKey'.

* Running the query with at least 25 other valid keys returns the 25 and ignores the one that does not exist. 

Are we missing something about how to query by key or is this a bug? 

Thanks