Sounds just like Erwinia infection. I am constantly inspecting my strap leafed Paphs and Phrags for this infection too. It shows up very suddenly and seemingly without any reason. I treat by promptly removing the affected tissue, even if it means the entire growth, right down to the rhizome. I spray the affected area with "No-Damp" and place the plant in a location where it gets a LOT of air movement. Usually, that means placing it on my potting bench, which is right in front of a 48" exhaust fan. If the fan is not running, I'll place the plant where it will get a good blast from the circulating fan. The next day, I will spray the entire plant with aspirin water (1 regular aspirin disolved in 1 litre of water). Most plants recover; some don't. It's disheartening. Prevention includes keeping your growing area clean, providing the best care possible (which reduces stress on the plants) and providing more air movement at all times. I know I don't have enough air movement and I intend to fix that by installing more fans....as soon as I can afford them.
Not sure if I should say this....because the "culture" here at ST has moved to be very anti-potassium (increase use of K-lite, etc.). However, I used to use a hydroponic feed most of the time (7-11-27), and it seemed to work very well for me. But, like many people, I was intrigued by the low K talk. So, I switched feeds and used ones with more moderate levels of K. Then, I began to get a LOT of this Erwinia rot...everywhere in my Paphs and Phrags. Finally, I went back to my 7-11-27 and the incidence of rot has reduced dramatically. It still happens; but, with much less frequency. Long ago, I came to understand that K helped make strong walled plant cells and that helped the plants to resist being infected by disease pathogens, like Erwinia bacteria. That's why I started using it. My experience with more rot when I stopped using it and then less rot when I went back to it has demonstrated to me that there's still more to learn about the benefits and drawbacks of a low K formula. I did notice that with less K, many plants had a growth spurt; but, what good is that if they are so soft that they get infected and rot?
I have also noted over the decades of growing that low calcium seems to invite bacterial infection. I've had plants that were cured of rot simply by watering them with Phostrogen, which is a high calcium fertilizer from the U.K.