Some rot causing organism has invaded. It may have followed a wounding of the stem or perhaps very soggy soil condition, or even sunburn on the stem. If you can cut the top off above the soft area it can be rooted in a moist growing media. Cover the top with a clear plastic to deter moisture loss and place the rooting plant in a bright area out of direct sun. Then remove the soft area on the "mother plant" and in time it will send out new growth to reform a new top.
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