I've got a Mac and an iPad on iOS 11 and I've got an iPhone on iOS 12. I've rebooted the iPhone several times and tried enabling and disabling iMessage in iCloud settings on all devices. On my iPhone I deleted most conversations so I only have a few left and on the Mac and iPad I have a lot of conversations so definitely not in sync.
[doublepost=1529460329][/doublepost]After fiddling for a while it seems to be working again! Not sure what fixed it but at least it works.