I think I figure it out. My fault again. The snake plant's meristems are dead, both the shoot and the leaf growing tips are no longer viable. It maybe due to inconsistent fertilization in the past, as I don't use commercial liquid plant food and the plants got starved for months in winter. So when the next
boost of nutrients is available, they can only expand leaf size or initiate new shoots.
I repotted and gather all the shoots with dead center in one pot and put young shoots in new pots. I will take better care and see how they will grow then.