First of all, if i make any mistake in English excuse me.
Download and Upgrade your OS to Catalina Public Beta 4. fn key and function keys on touch bar are working.
But i want to mention a new bug which is function keys default on some apps.
If you want to use functional keys by default on some apps, you could add that app from settings > keyboard > shortcuts > functions keys.
But here i can tell that my saved apps in here are not display function keys by default. I need to press fn key to get them.
Device: Macbook Pro 2018 15' w/ Touch Bar
OS: Macos Catalina Public Beta 4 10.15
EDIT: Function keys are working as default but after one-time fn key press. On Touch Bar latest status is saving and not refreshing app-by-app. If you switch app from fn key default to app controls default app. You need to press one-time only fn key to refresh