There is an actual Kentico Setting under URLs and SEO that is says "Redirect pages to main extension", what is the value of that? If it's checked, it will redirect to the primary page url, if it is not checked it won't.
Also Alias's have a culture setting, it can apply to all cultures, or a specific one. If you have a specific culture set on the Url Alias, it will go to taht culture always, so you may have an alias of
fr-CA/folder/page but if the culture is set to
en-US on the Url Alias setting, then it will give the english page.