The possible reason could be Google analytics JavaScripts. If you use them please try to remove them and test the behavior. Regrettably, they do not work well together with our multilingual system.

Another reason could be that one culture uses the same custom URL (Properties -> URLs) as document Alias path. Please check the custom URLs of all cultures, all of them should use different strings. Alternatively, you may use empty string (Use custom URLs is checked) for all cultures. Then the system will handle cultures correctly.