Some podcasts missing from Smart Playlist
This appears to be a bug in iTunes 12.3.1.23.

I created a similar playlist to the one in my question, but I replaced the "Rating" condition with "is equal to one star".

This new playlist contained all the "missing" podcasts. However, the "Get Info" for many of these podcasts indicated that the rating was NOT one star. These podcasts showed in the playlist with a grey star. Podcasts, which were actually rated with one star, showed in the playlist with a black star.