I’m having fits with this as well. About a week ago I updated both my iPhone and iPad to iOS 11.4 and enabled Messages in iCloud on both devices. Everything appeared to work fine and both devices synced perfect.
Then yesterday I had a Genius Bar appointment about some issues I was having with my iPad screen responsiveness. The tech suggested erasing the iPad and restoring it from an iCloud backup. So I did this once I got home.
Everything restored properly, except for Messages. For some odd reason, I have everything from the beginning of time until May 12th of this year, and then I have all messages that came in from the day last week when I enabled Messages in iCloud until today. But from May 13th until last week, all those messages are missing from my iPad and after leaving the iPad plugged in and connected to WiFi for a full day, none of the missing messages have downloaded.
I do have one question.... I see above some people referring to progress bars showing the downloading of messages. Where exactly on my iPad would I see such a progress bar? When I open messages I don’t see any progress bar anywhere.
For now I’ve just turned off Messages in iCloud on both my iPhone and iPad. My iPhone currently contains my complete set of messages, so hopefully at some point I can re-enable Messages in iCloud and fix this mess. I did call Apple tech support and they were of no help. Their suggestion was to try restoring my iPad from an earlier backup. But if Messages in iCloud is storing the messages in iCloud, then they wouldn’t also be stored in the device backup, would they? The tech seemed unsure.
Did you figure this out? I just did a reset on my wife's I phone and no messages after May 12 are showing. The weird part is, the time for the last message received or sent is updated when you first go into messages but the messages are missing from 5/13 onward.. .