spider middleware process_spider_output() doesn't capture item yielded by spider request errback if it was called by downloader

spider middleware process_spider_output() doesn't capture item yielded by request errback if this method was called after an error raised in downloader/downloader middleware.

However, it does capture items yielded by errback if this method was called after an error raised in spider or spider middleware.

Is it a bug or a design decision?