I text mostly with iphone, still I use messages sometimes on desktop/ laptop.

Usually what delete/ trim off iphone messages, reflect on desktop/ laptop accurately, but no more. And list getting very long and unwieldy.

I brought it up on phone last with a good Apple tech, and we peeked into desktop icloud seem ok right things checked, but never resolved. And laptop has joined the desktop in long list of messages that were deleted from iphone months ago. so gotten worse, thanks!

Messages has got a "Keep messages: 30 days/One year/Forever" option in its prefs.

What's your setting?

Good call Artie! Amazing Apple tech, who was quite good did not mention. He went to all sync/ icloud stuff, not relevant, your tip solved it.