Missing text thread

I worked with Apple Care for at least 45 minutes, launching in safe mode, checking multiple settings, shutting down, relaunching, working on settings in iCloud and more. There was no change. It must be a glitch. To answer your question, sending a text from the thread on the iPad or iPhone simply appears on my iMac as the latest addition to the thread and doesn't make the whole previous thread appear. All threads going back months or even years are on the iMac, except this one which prompted me to call them. We've had several glitches on the iMac since installing Monterey and this is one of them. I'm on a late Dec. 2015 model and it's just a bit too old to work ideally. Or perhaps Monterey does still have glitches. I've certainly seen issues people have had with it even on newer computers.