You have to go to Customize on your homescreen in the left hand upper corner - the tiny „+“ after you edit your homescreen.
There is the option, to turn all apps to darkmode if it’s not working automatically.
If you want to have it switch automatically try to hard reset/force restart your iPhone.
If it’s not working afterwards and you want dark apps, just turn it on in the customize settings
Edit:
Tried it myself on Beta 3 and it does not change automatically, I will use the feedback app to report this bug to Apple.
Feel free to report it as well.
Here’s the Feedback Report ID so you or anyone else can copy it and tag it to their report of this bug: FB14338276