The phenomenon is reproducible!
Cassette is 12-25, so the bottom cogs are 12-13-14-15 etc... The chainrings are 34-50
I don't run 12-34 because of habit, but I can run 13-34 with no apparent problem.

So, when I am in 13-50 and I shift to 13-34 (in other words, when I'm in the large chainring and the second cog, if I shift to the small chainring), I get the 'float' problem and the chain does not immediately engage with the small chainring. It floats between the 2 chainrings and I sipn.

I can make this happen any time I want.

I removed FD and cleaned it thoroughly - it wasn't that dirty, and it moved smoothly before cleaning.
I also cleaned the cable and housings and the cable did and does run smoothly. Put it all back together and now it's REALLY SMOOTH (campy can be so fine!). But chain still drops between the rings as mentioned above.

mouse ears: If I am VERY AGRESSIVE about shifting to the limit, I can make the chain jump onto the small chainring. Problem is that for FD to travel the entire range between the limit screws, I need more mouse ear clicks than I can get in a single swipe, so the shift is sort of a 'double swipe', and it's inelegant. I may be able to reduce the range between the limit screws without putting the shifting off, so that I can get the shift in a single swipe. Then it will be a quirk of my bike, and I can get used to it.

What do I need to know about Rear derailleur capacity to absorb slack in the chain? This seems like a reasonable hypothesis. When the chain finally settles on the small ring, it has a definite 'snap' to it.
How do I check chain ring alignment with 'timing marks'?