Neither of these problems is a regex problem and its not about being more or less strict. Once the series title is parsed it needs to match a series, either by matxhig the series title or an alias (naming exception).
In the case of The Flash being an alias for The Flash (2014), you can’t just drop the year, this would conflict with the original series with that name unless they are released as a different name (which appears to be the case here).
In the case of American Horror Story, American Horror Story Hotel could be a completely separate series, its not, but could be and unles Sonarr knows about every series available it couldn’t guess that it’s really the same series nor do we want to guess that it’s the same.
The reason it works when you drop it in the series folder is because Sonarr doesn’t care about the series title that is parsed when a file is in a series folder, it already knows the series. That means if you drop a file for a completely different series into another series’ folder Sonarr will import it and is a big reason why Sonarr requires a series folder per series, not one folder that contains multiple series.
A false positive is in fact a dangerous thing, consider this, if you had all the files for a particular series and Sonarr incorrectly grabbed a release and replaced one episode file, would that be a big deal? Its a pretty easy problem to reverse by regrabbing the proper release, but what if it kept happing? Or what if it happened for every file in one season? Or every file in the entire series? There are thousands of Sonarr users, so it wouldn’t take much for this to be a big problem and the reason why its handled as it is.