Someone in another thread which, oddly, I can't find (*) now that I need it, had mentioned that he had seen his maggots being carried away by hungry dwarves as food! This would explain why I had several live maggots which were tagged as owned by a dwarf. They were probably uneaten food (probably didn't know how to pick them up since they're in a cage) that he still owned.
Are dwarves supposed to eat raw, live purring maggots?
I do hope this is actually a bug. Given the cost for obtaining a maggot (generally three stacks of meat, a fair bit of hauling time, and a few bats you can't get rid of), this is a severe pain if it's how things are intended. It would be a bit more reasonable if trappers were smart enough to split a stack of 9 meat into a stack of 8 and 1, and only bait a trap with the 1 meat.
(*) The search feature for these forums is decidedly painful because there's no way to -search all forums-. It is very tedious to go back and forth)
P.S. Pardon the post full of complaining. I'm delighted that there's enough detail in the game for such complications to arise. But considering it used to be a bug that live maggots would get automatically butchered, I'm guessing there's a lingering issue with them being treated as food, rather than an intentional in-game annoyance.