I've had it happen too but it's never done more than kill a leafIt's a rot. I would say pseudomonas.
It can start anywhere on the leaf.
I have that on some plants at the moment, unfortunately. usually Maudiae type.
As you say, if you leave it, it will kill the entire leaf.
Best to cut off the affected area as soon as you see it start.
Usually that's the end, but it can start from the cut end in just a few days of at times even months later.
I hate it!!! Lost a couple beauties to this disease.
So far, disease free Paphs have been bulldog hybrids.
That's possible, the area in in is notorious for bad water, that and Mosaic just accidentally dumped a ton of chemicals into the Floridan aquifer so it could be even worse now. I'll try giving the plants 50 tap 50 distilled, see if that helpsI've experienced that from tds burn