This is not a deficiency. It is almost certainly anubias disease which rots the rhizome and the base of the stems, then later attacks leaf tissue. It seems to spread by contact with infected parts of the plants. It is potentially caused by a slow moving plant eating worm (though this needs to be verified by microscopy). There are no known cures, but aggressive pruning of diseased areas can save plants if you catch it early. Be sure to clean the blade between each cut or you will infect healthy tissue and cut into the "healthy" areas around the infected parts or the disease is likely to keep spreading.
The only other ways anubias are damaged like this where the stem becomes mushy is by improper shipping (wet conditions for too long, or when temperature is too high or low during shipping, this promotes opportunistic fungal attacks).