Image Capture - Detect Separate Items - Crops from previous scans are not cleared with a new Overview scan

We're scanning a bunch of photos using Image Capture and a Canon MP480 flatbed scanner. We're using the "Detect Separate Items" feature where after an Overview scan, Image Capture automatically adds crop regions around each individual photo. Then, Scan causes the scanner to do a separate scan pass for each photo.

The problem is that after doing an Overview scan, the regions aren't cleared from the previous scan. So, for example, if we put 2 large photos on, Overview, and Scan, we end up with 2 files cropped properly. But if we then place 3 smaller images on the scanner and Overview, the window shows those 3 images with crop boxes around them, but still shows the outlines from the 2 old photos, which are now overlapping parts of the new images. If we then hit Scan, the scanner scans 5 separate regions, producing 2 images using the sizes of the previous ones and 3 new properly cropped photos.

We can get around the problem by clicking the "Auto selection" drop down and clicking "Detect Separate Items" again, but in the past it's done this automatically (i.e. doing an Overview scan would always clear the crop regions that had been added and detect completely new ones) which is the desired behavior.

What would be the cause of this odd behavior? I don't see any settings to keep old items.