I am not aware of a change in the behaviour of this, but from version 2.1.3 Thymeleaf is handling HTML entities using unbescape library, maybe the escaping issue is related. You could try version 2.1.2 to see it this is the reason.
Anyway, maybe this is the expected behaviour, as the ampersand character must be escaped in HTML. If you don't want it to be escaped you could use th:utext.
Other option is not to use the HTML entity in your properties file but the unicode escaping, like \u00c9 (I thinks most IDE will do this automatically for you).