Interesting issue.
I can confirm it happens on Chrome, Chromium and Firefox.
This must be a javascript issue. Each time you select "latest", something somehow changes the value of the select to null, which cause the empty box. This does not happen on other installs of Flarum (Here on discuss selecting Lastest sets the select value to lastest)
From the javascript bootstraping code in the footer of your website, it appears you are running a pre-beta6 version of Flarum. You should probably make sure your forum is up to date before anything else ?