Exactly the same, it started with my iPhone 14 Pro, when downgrading from 1TB of storage to 512GB because of price hikes that year. Ever since it's been a mess with no solutions. I spent the same amount of time with Apple Support, maybe more, so many of them "senior" advisors being clueless or rude because they had no solution but tried to pretend it wasn't an actual issue. So many of them promised to follow up but never did.
I have 205GB of iMessage data in iCloud, and never able to get it on my iPhone no matter what I try.
When going into iPhone storage, the information I see is false. It says iMessage is only a few GB, but if I turn off iMessage in iCloud, all of a sudden I have 100GB of messages data. The storage appears different if I connect to my Mac in Finder vs what I see on my iPhone settings.
On my Mac I'm able to see all photos/videos in the Messages app, although it only downloads around 150GB of data, not the full 205GB.
On iPhone the behaviour changes each time, but overall the "download" button is useless, it says it downloads but it doesn't, or maybe it does in the background but nothing appears in photos. The number it gives is also false, usually 30% or less of the actual amount of photos I've exchanged with said people. If I scroll indefinitely I may be able to find some photos, although some never want to download.
It's so frustrating to have been paying for many years for that iCloud+ 2TB storage and I can't get a decent service. Also frustrating that WhatsApp ALWAYS backs up and restores content perfectly, each year I upgrade, while Messages remains a mess.
I don't know what to do anymore.