The retention can be a bit unintuitive, since it only looks at the time difference between two versions. Since your backups seem to run at different times (3am, 8am, 12pm), there might be only a 14h difference, which is less than one day and might cause a version to be deleted.
As you run the backup only once a day, you can keep every version for 7 days with 7D:U.
See also: