My guess would be your tuber is rotting despite outward appearances to the contrary. Can you cut off the root end of the tuber and check the inside?

I just had this happen to what I thought was a robust little plant, until it wilted almost overnight. When I lifted it out of the pot, almost the whole tuber was rotted and all got tossed.

If there are roots forming higher up, you can cut the tuber below them and replant, hoping for the best. Otherwise a cutting might be your only hope to save Zorro.

That's my two cents.

Poochella is right- sounds like your tuber is rotting before the plant has enough feeder roots to sustain it. If you did a cutting at least you wouldn't be out a plant- & if the plant recovers, you'll then have an extra Zorro. Do the cutting so you have 1 set of leaves to nip off & put underground & one set of leaves above ground. That'll leave a couple of sets of leaves to keep the poor thing going - sort of like you've topped it.

I did take a cutting before going in to investigate. I cut the tuber at the root end as you suggested, Pouchella, and the darn thing looked good with some very healthy feeder roots at both ends! I ended up "dissecting" it, so I lost that tuber. But in the end I think it was a good learning exercise.
The cutting was taken over 12 hours ago and still looks poorly. However, the cut turned out to be at a hollow node. IÂll cut it up further as you suggested, Plantlady, and hope for the best.