I have the same issue. I first noticed iMessage syncing was not working properly in iOS17/iPadOS17. After several revs to ver17 and no fix, I complained. After ver18 was launched, I updated both my iPad Air3 and iPhone 14Pro. iMessages continued to be out of sync. There was a note in the iCloud settings tab that stated the iPad had not been synced with iCloud server for three days. The iPad had been used many hours, logged into my Apple ID, over those three days. Why isn’t the iCloud server connecting and syncing with client devices? Today I updated a pdf on the iPad and saved it to a shared iCloud file. I went to open the file on my MacBook Pro and the file did not exist. Since there is no method that I can suss out that forces a device OS to sync with iCloud, I was stuck. I used Airdrop to move the folder over from the iCloud server to the MacBook local drive. Once I did so, after just a few moments, the iCloud server synced with the MacBook and created a second set of files that were time stamped from before my Airdrop maneuver. That earlier time stamped folder had become an “older version” of the file I had modified on the MacBook and therefore required deletion.
So for me, I still don’t trust sending iMessage on the iPadOS cuz they may not appear on my other iCloud devices. But I do know for certain after today (1/23/25) that iCloud is not syncing properly with either my iOS18.2.1 iPhone14Pro or my iPadOS18.2.1 iPad Air3 or my MacOS Sequoia 15.2 (24C101).