Yeah its a strange one. Are you sure you haven't fed anything else? Like MOAB, shooting powder, or some kind of MKP? I fed with MKP mid flower, and I also saw my stigmas (often miscalled pistils (often misspelled pistols)) disappeared. At very close inspection I found that they had browned and receded. I saw a great deal of swelling, not like yours, but weird. Stigmas came back a week or so later, and everything finished ok, but with some foxtailing. This was outdoors.
I wonder if it's to do with exposure to light? I mean, it seems obvious light is involved somehow, with it happening lower in the canopy. Can you drop a CFL down near a lower bud and see what happens?
Airflow?
It's a good mystery. By the way, there are very few insects, rare to encounter, that will solely eat stigmas, so don't go down that rabbit hole.