I know this because I am meticulous about backups and these particular files are encrypted private records. I don't trust software but I especially don't trust that backups will correctly handle sparse disk images since they are spread out over hundreds of files; so I always keep the output of mtree in that directory for the current version. I had checked the mtree file, edited some of the files contained therein, and regenerated it the night before; SOME but not all of the sparseimage subfiles had rolled back to October. Changed timestamps, changed hashes, the image did mount but the enclosed files were from October (they have date-stamped rows).
So when I went to open the disk image, the next day, and ran mtree to test the directory first, it noted the changes above.
I moved to icloud drive about two years ago. I'd previously had some problems with Apple's sync blowing away some notes in notes.app so when I moved everything from Dropbox (where my paid account for 7 years never lost or corrupted a file to my knowledge - and my knowledge is good because this mtree habit I just described goes back a decade!) to iCloud, I kept this habit up, and good thing.
To this day I don't know how it decided October or ended up replacing them. Maybe there was some local cache, I don't know. I did not revert the file in iCloud and nobody else has access to my account. Further, it only partially rolled the image back.
I had never had a problem up to that point, but I will say, with storage, once is too many times. Back to dropbox.