"I did use Picasa a long time ago, so it is possible that the photos at issue had their faces tagged before importing into Lightroom."
Picasa followed the same industry standard as LR for face tagging, but that standard didn't specify how to use keywords. So when LR imports Picasa-tagged photos, it imports the face rectangles along with the labels under the rectangles but not the corresponding keywords (because Picasa didn't assign industry-standard keywords).
If you're experienced with command-line scripting, you could use the free Exiftool utility to copy the face labels to keywords. But beware the learning curve for Exiftool is steep. There was a thread in these forums from several years ago that gave details, but Adobe in its wisdom discarded older threads when it migrated to a new forum platform.
You could also check out the Picasa Face-Recognition Import plugin. The author has an excellent reputation. It's not clear whether it reads the data from the Picasa database (which must be still on your computer) or from metadata Picasa stored in the photos.
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