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These articles should help with all your onion growing: https://www.gardeningknowhow.com/edible/vegetables/onion/

Isn't that interesting? Sounds like a fun project for a kid's experiment. I don't have time to look up specific scientific explanations, but I suspect it has to do with the composition of the sap in the scallion stem, which is not only slightly sugary because of the carbohydrates dissolved in it, but also has something a bit sticky or oily in it, and then the addition of the gases that are also in the sap, and the fluid pressure which moves the sap through the plant pressing against the liquid collecting at the site of the cut, and forming a bubble.

I believe it is either a Pregnant Onion plant or a Climbing Sea Onion, neither of which are true onions. Here is more information:

https://www.gardeningknowhow.com/houseplants/climbing-onion/growing-climbing-onion-plants.htm

This is an issue caused by a lack of phosphorus in the soil. It may be too late to reverse these onions (but it does not hurt to try), but add bone meal to the soil to increase phosphorus. At the very least, this will help fix the soil for future plantings. Also, have the soil tested to see how severe the deficiency is.

Ericaceous compost is acidic in nature and a good choice for acid-loving plants like azaleas and blueberries. While some acidic soil is tolerable, this particular compost may not be suitable for use on oinions as they are sensitive to overly acidic soils, growing best when the pH is between 6.2 and 6.8.

Here is a blog that will interest you where somebody took a sprouting store-bought red onion and planted it with success. It apparently can be done.

http://www.anktangle.com/2011/03/growing-sprouted-onions.html