Personally, I would break open the skin on the rotting area and expose it to the air. I have found that this kind of rot keeps getting worse if ignored and will keep working it’s way up the stem. This kind of rot seems to thrive in anaerobic conditions and once exposed to the air, it dries out and quickly heals itself. I open up any kind of rot that develops on my dragonfruit. And less water. Works for me👍.