We're sorry for the confusion. 6.5.6 is the latest version of X so that's not the issue.
I searched for the reason why this happens and this third-party forum is the closest to what is happening in your site.
Based on the thread, though the issue comes from our theme, technically, this is a bug with Safari. Specifically, this has something to do with using a third-party font service like Google Fonts. This is why the Select input works when you switched to the Twenty Nineteen theme because it does not have that setup. In X, I see you're using a Google Font in Theme Options > Typography.
Would you mind switching to a System Font like Helvetica and make sure nothing in the page uses Google Font just for testing?
Either way though, we can do nothing for this case as this is a browser bug. We could not limit our users to use system fonts only.