Same for me.
After updating to iOS 15.6, my iPhone 12 will not provide continuous speech-to-text dictation. It cuts off at around 1 minute and 40 seconds, consistently, whether I’m on WIFI or cellular.
What it seems to be doing is right at 1:40, text starts disappearing as if Siri is re-interpreting what I wrote, it puts in its replacement text, and then it stops dictating.
I just tested this with airplane mode on. It listened for 1:26 and then shut off and stopped listening.
So far I have tried disabling Hey Siri, all Siri settings, and Settings > General > Keyboard > Enable Dictation, deleting my Siri history… then restarted my phone.
No matter if it’s in airplane mode, WiFi, or cellular, Siri will not continue dictating in the Notes app for more than 1:40, and it shuts off sometimes earlier than that.
I don’t need a link to contact Apple Support (I was a ten year Apple employee), I am simply chiming in that I have this issue as well.
Very frustrating, glad to see I’m not alone.