Hmmm, maybe your new lights are too good, or you were leaving them on too long. I'm not familiar with those varieties, but if they were long day types, and the lighting period was long enough that could have switched them to bulb mode even if they were still small.
I've had success trimming back the tops at the point of the final xplant to outside. Helps balance the leaf load to the reduced roots, but also might cause the plant to "rethink" bulbing for a while, IDK. I generally grow out my onion starts in natural light, which is pretty limited in Feb-Apr, so bulbing isn't an issue for me.
Let us know how these do.