I've recently updated macOS to Sonoma from Monterey. I noticed that when I used rsync --av, it would keep copying the same files over and over again, even when I just reran the same command. This is occurring when I'm rsyncing files from the M1 Max Macbook Pro to a Samsung external hard drive, which is formatted exFAT. (I don't recall this being an issue on Monterey.)
I've had the same problem with rsync version 2.6.9 and with version 3.2.7.
If I use --size-only , this resolves the problem.