I had missed that the Capture Area Device needs to have the Flag item set. I don’t know why, but I just assumed that the device would always capture flags.
The documentation does not include the Item List setting in the table of Capture Area Device options. Above the table, the document says:
Once you have the Capture Area devices placed, customize the options for each as shown below.
However, the Item List setting is mentioned above that line, which I missed.
I suggest that the document be updated slightly to read:
Once you have the Capture Area devices placed and the Flag item assigned, customize the options for each as shown below.