I'm not sure about the differences between beta.15 and 16 search wise (@askvortsov would know more on that). But what I can say is that MySQL and MariaDB seem to always have issues indexing and searching non-Latin characters. Especially things like Japanese, Chinese, etc. This is a limitation of the database itself and something we intend to resolve in the future via search drivers. That way you can use a better search index system for your language/size forum like Elastic, Algolia, etc.
