In an attempt to save them, I transferred a few remaining plants into a pot with rotting Ketapang leaves + coconut husk, hoping the the pH of the substrate would then be lower to aid in their growth. However, these plants melted away too except for...
... one leaf with petiole already melted away. This particular "last leaf standing" managed to remain green and healthy looking for more than a month already. Below is a photograph which I took yesterday to show its status, it is still looking good except for the petiole connection area which has started to turn yellowish.
Weird?