I restored from an iCloud backup of my old phone (iPhone XS Max with iOS 18.x.x latest at the time) to my new phone (iPhone 17 Pro Max w iOS 26.0.1 latest at the time) and after using the phone for a few days I discovered that some conversations in Messages were incomplete - missing months of posts.
After spending hours on the phone with a senior tech specializing in Messages app, there was no resolution. Only option was to erase the new phone and try again to restore from iCloud. I didn’t want to do that since I had been using the phone for several days by then and did not want to have to redo the settings and other things I had done on the new phone.
So, all things considered I decided my best option was to accept the situation and move on. Fortunately, I didn't have anything too valuable in messages.
FYI. During my first Setup Focus session we tried doing a direct device to device transfer and it was a fail. Thus, my second Setup Focus session we used the iCloud backup which was made immediately before the Setup Focus Session.