Well, per Tech's suggestion, I ran a boot-time scan, and sure enough, ALL files were scanned; no "Some files could not be scanned" message. But after that, I immediately ran a Quick Scan with Windows fully loaded...and the restore files again could not be scanned. The specific message under the Status column in the log is "Error: Archive is password protected (42056)".
I realize this is a nuisance, not a serious issue, but I'm suspicious when a program behaves one way for almost 2 years, then suddenly starts behaving differently on a consistent basis. I will try zapping all the restore points, create a new one, then run another scan to see if it scans the new RP or continues to skip it.