Hi RobSpo56,
welcome to the Apple (user-to-user) Community!
Re: "iPhone 16 randomly not doing speech to text"
If you haven't already, you may want to try updating your iOS version to see if that helps:
(NB: It is always wise to back-up your iPhone data before updating):
About iOS 18 Updates - Apple Support says iOS 18.1 is the latest version:
"This update introduces the first set of features powered by Apple Intelligence"
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If you are already. using iOS 18.1:
You may want to send feedback to Apple. They will likely not reply, yet, if others also have the same issue, it may be addressed in the next update.
ie: To provide feedback about an accessibility feature on an Apple product,
you can email the Accessibility team: accessibility@apple.com.
All the best :-)