is it observed during early-stage only? If yes, then I believe it is due to the excision when you were doing surface sterilization or subculture...
It is very less likely due to the malformation caused by plant growth regulators but it could be the case...
I do observe few leaves splitting at the early-stage of the culture...you may try to add activated charcoal to suppress the supplements so that the plants won't get any stresses
You're right. After sterilization I added active charcoal and I didn't see the problem. but When I eliminated AC in the first subculture the leave began splitting.