Nematode, I had the same thing happen with bulbils on cut scapes in water last year - they were basically sprouty. Also saw the same on some that were bulb attached in the field last year - it was wet. Moisture for sure the cause.
I dried the lot of them, but can't say whether sprouty ones that dried were afterwards good or not...
they were put in with the rest, some came some didn't.
I did take a whole "punk" of tiniest sprouts, cut off the stem and planted the whole thing into a pot for curiosity, and kept in greenhouse instead of curing and planting outdoors. It ended up producing some little bulbs - no bigger than bulbils but pretty much came from nothing.
If you want to get every bulbil for sure you could pop the sprouted ones and plant them now - but then protect them over winter.
This is my bulbil setup - some flats outdoors heavily mulched for winter, had to weed these once. Not harvested yet. But lots of them came, from bulbils off of cut scapes. Drainage is key for flats outdoors - I think some of the misses in the smaller tubs are due to bad drainage, freezing and filling with water to overflowing with ice a couple times this winter.