A touch of light frost can damage the young tender growth on the very top and cause a mushy top. Excess moisture collecting in the top during a damp cool period or heavy dew can also cause it. New, young and tender growth is most susceptable, that's probably why the larger, further along plants were not affected. You should remove the damaged part or top and keep the rest of the stem and leaves to replenish the bulb for next year and it will be just fine.