In the meantime i've actually run into this problem as well. 2 months ago one of my phals has arrived with signs of rot right from the vendor. This is how it looked like on arrival:

The spot on the leaf tip developed further right away so it had to go, i cut it off after 2 weeks. Looked like this right before the cut:

The leaf tip's been okay since and the spot on the stem was stagnant so i thought it was all right. Then it continued to darken just a few days ago and appeared on the bottom part as well...
I instantly knew it needed immediate attention:

So i cut the leaf as low as i could.

The operation took place 2 days ago, again, i thought we were in the clear. Today morning i've seen another brown spot on the base of the next leaf up on the stem..

So it's operation time again this afternoon. Wish me luck...