iOS16 is still BETA software. The full, working version of iOS16 will not be released until September / October.
You have obviously voluntarily signed-up to the BETA program, so you know the risks: some things work, some do not.
It is possible that the dictation function in the BETA version does not fully integrate with third-party apps which will not themselves be configured to work in iOS16 because it does not 'officially' exist as an OS.
That said, you can still report the things which do not work to Apple: that is the whole purpose of being a BETA tester.
It is not there to give you early access to a fully working OS ahead of everyone else.
(EDIT: This is where the public BETA program fails in its intended purpose, in my opinion; I see these same kind of posts repeating themselves every year).