Here's a story from the early days of iOS/iPadOS 14. After updating my devices to said operating system version, I noticed at some point that my favorite locations (in my case: home and work) in Maps were gone.
This happened to my iPhone and iPad simultaneously. I tried various things to fix this issue, but to no avail. For example, I tried declaring my home address a favorite and the menu item for favorites did: nothing.
Shortly before deciding to give up, I remembered that I have another iOS device that–at that time–was still on iOS 13. I tried, and sure enough: the favorite locations showed up in Maps on iOS 13.
Not only that, they instantly also showed up on my iOS 14 devices. I can tell because I had Maps open on my iPad when I launched Maps on the iOS 13 device.
My best guess is that this issue was caused by some anomaly in iCLoud. I'm fully aware that I'm not the only person getting tricked by iCloud. In contrast to some other people who suffered more or less severe data loss, I've been fortunate enough to just lose some bookmarks (and got them back 100%).