When rescanning, and using archival Fast Backup, it may not copy files with the archive attribute set
If a Rescan is done, either by clicking the Force Rescan button in the profile setup, or by other means, then when the profile is next run it will scan both the Source and Destination, compare the files, then copy new and changed files. This has a side effect with archival Fast Backup in that it will not - during such a Rescan - copy a file (even if it has its archive attribute set) if the Source and Destination files are otherwise identical (the only difference being that the Source file has the archive attribute set). If the profile is run again (so it is not now a Rescan) then it will scan the Source (but not the Destination, as expected), see that the Source file has the archive attribute set, and copy it to the Destination regardless (because of the archive bit).
This is because the decision process/basis for archival Fast Backup is different during Rescan and non-Rescan runs. It is related to the fact that if run-1 (the set-up run, i.e. in Rescan mode) reacted only to the archive bits, and you had Source files without that bit set, they would not be copied on that first-run (thus making your initial backup incomplete). Naturally, such files would never be copied on bit-based runs afterwards either, unless something changed their archive bits later. Hence a Rescan reacts to standard differences-detection (to hopefully achieve a full base-line backup), and the intermediate Fast runs react to the archive bits.