I think that is normal with stem plants, it happens to mine too, especially the hygrophilia. These plants either need pruning to bush out, or do as suggested with stem plants in the pruning sticky. It suggest you can remove the plant, cut to size, discard the bottom half which by now will have more roots that you wanted, and re-plant the top half of the plant with some of the new roots at the bottom.
Hope that helps
Squid