Cannot restore from Time Machine back ups after changing operating system

I have an iMac which was running Ventura. Foolishly, as it turns out, a few days ago I accepted the invitation to try the Beta Sonoma OS. It proved difficult since Lightroom in particular repeatedly crashed on opening. So I decided to revert to Ventura. I assumed that I would be able to restore from my Time Machine. However, when I restored the OS using internet recovery, Catalina was installed. I upgraded to Ventura. But now I want to restore my machine to how it was before the fateful decision to try Sonoma. But Time Machine does not show any previous states from which I can restore. The TM disk holds back up files going back six weeks but for some reason the TM app doesn't seem able to access them. What am I doing wrong? Any help gratefully received.