Well, I tried Michael's suggestion, where I selected a sound and cranked up the envelope release to the max, and then tried to play 32 notes to see if I could hear them cut out. It's still heard to tell, because even at maximum release, the sound fades out naturally as I play more notes. But what I did was to start out with high notes, playing chords or arpeggios, and then quickly did the same thing at the lower end of the keyboard, figuring that it would be easier to hear the high notes cut out. I did it with the string sound, and it seems like the original notes were playing even as I played more than 16 notes, so it seems like there is truly 32 notes of polyphony. However, I need to try it with other sounds.
If there really is 32 actual notes of polyphony for all sounds that don't turn on the dual voice, then maybe the reason I was having notes cut out had to do with a small amount of release causing chords to momentarily overlap, causing substantially more notes of required polyphony at that moment that the bass note cut out.