This is also happening to me and it's very frustrating! I select a piece of text to read and it stops after the first or second sentence, sometimes it reads a paragraph or two, sometimes it stops immediately or after two or three words.
I used to be able to read whole pages without any problems or random stops.
The only things that seem to help is turning on the Audio Background sounds and the Spoken Text Controller (which I prefer off). Somehow with these on it reads a little bit longer but I still can't have it read whole pages anymore.
Could someone escalate this at Apple? This is a major accessibility issue!
It started a little while after I updated to Sonoma, but other than that I've made no other big changes to my system recently.