I got my launcher updated to the latest version when I found out that on the "PLAY" button 1.13 and all its development versions I had downloaded before required me to "download". I've checked the .minecraft folder and the versions are indeed there, but the button keeps requiring me to "Download xx".
However, the file which is re-downloaded is only realms-<version>.jar, which means only 1.13 stable version actually re-downloads something. Although the development versions also display "Download <version>", they don't re-download anything and act as usual when "PLAY" is pressed. EDIT: What happens now is that only stable released versions (and not only 1.13) re-download, and what're redownloaded are their language files (xx-xx.json/lang).
EDIT 2: Now only new realms-<version>.jar is re-downloaded for stable versions.