When I was a kid we had a patch of scallions...
I would almost bet you had what we called winter onions. They were mild enough in the spring but became hotter as summer progressed. Ours reproduced by producing small sets on the top of the stalk, then when the stalk matured and bent over, that set would settle on the ground and grow into a new plant. Then the mother plant died down and early in the fall you would have a repeat of the young onion, which again was tender and not hot.
I couldn’t tell ya. Don’t remember that well. but it doesn’t seem possible to me. They grew so dense that there was no available space between bulbs. It SEEMED like they were reproducing underground, and just splitting off new roots.