Strange Nep problem

[Ifurita]

Recently I've been having a strange problem with some of my Neps and I have no idea what's wrong. The affected plants just suddenly wilt overnight, with many of the leaves turning black/brown and the remaining green ones curling up. Usually the leaves which don't go black/brown are the newer ones near the growing tip.

At first I thought the plants were rotting at the root level, until it happened to a large plant which was happily basaling and when I dug it up, I found that the roots looked healthy.

The strange thing is that this happens to isolated plants, almost one at a time, with all the other Neps around them unaffected. It has hit both highland and lowland species and I can't seem to find any pests at all on the affected plants, except for one(it has some scale) but that one is the only survivor so far! I have been wondering if it could somehow be spider mites, but I can't find any on any of the plants, nor is there any of the webing at all.

The only survivor so far is a large sibu x hamata, which I quickly repotted and flushed with water. The roots looked healthy, but most of the main vine had gone black practically overnight, with the remaining growing tip and 1-2 leaves very wilted. Two small basals were also somewhat wilted. The plant is still in shock, but there was no additional damage the day after, nor has the damage progressed any and it looks like the plant will probably recover.

All other affected plants were smaller and the damage affected practically all of the plant body, leaving 1-0 leaves and a growing tip, which were dead within a day or two, probably due to the shock. All affected plants were also affected one at a time, with between a few days to a few weeks between cases and the plants come from various parts of my growing areas, with only isolated plants here and there affected and affected plants almost never coming from the same parts of any area at all.

Can anyone shed some light on this? Has it ever happened to anyone before? I've never seen this kind of thing happen and I can't seem to come up with anything, pest/fungus/growing condition that can do this kind of damage and has this weird spread pattern. I've in fact been wondering if its my maid who has been randomly poisoning my plants with who knows what everything she's pissed at me, but I don't want to jump to conclusions...