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I have answered on the original, but just in case, I will repost my answer, here:

Iron chlorosis can be common over the winter. If you have given it feed according to instruction, then it will not be time to feed just yet. Rather, if your area does get quite a bit of rain, then I suspect that this is the culprit.

Root suffocation can cause poor nutrient absorbtion and rot. Make sure that the container has plenty of drain holes to allow all extra water out. These plants will not want to be saturated, constantly.

You may think about using a fungicide, as well. This can help kill or prevent any infections that are inherent to wet soils. Here is an article that will help you with fungicide use:

https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm

This article will help you to grow Passiflora in container:

https://www.gardeningknowhow.com/ornamental/vines/passion-flower/passion-fruit-vines-in-pots.htm

Overwatering can cause problems. Also, in dry weather, thrips will feed on the fruit, causing it to wrinkle. It also can be caused by sucking bugs, fruit flies or a boron deficiency. Here is more:

https://www.hort.purdue.edu/newcrop/morton/passionfruit.html

https://edis.ifas.ufl.edu/pdf%5CHS%5CHS139700.pdf

Only a few varieties of passion flower are winter hardy in gardening zone 6, including Maypop and purple passion vines. If you have one of these, you can plant it outside. Planting it in the early spring is the best option, but it is possible to plant outside now if you make sure to mulch the plant to protect it during the winter. Here is more information:

https://www.gardeningknowhow.com/ornamental/vines/passion-flower/preparing-a-passion-flower-vine-for-winter.htm
https://www.gardeningknowhow.com/ornamental/vines/passion-flower/transplanting-a-passion-vine.htm

Do the baby caterpillars not eat the plant? I understand they won't if it isn't the right plant. Or do they just disappear as if eaten by a predator? It may not like that cultivar of passiflora. I read it is not native.

Here is info about passiflora species:
https://www.gardeningknowhow.com/ornamental/vines/passion-flower/passion-flower-varieties.htm
Here's an article about passion flowers and their associated butterflies. it mentions passiflora incarnata.
https://sfyl.ifas.ufl.edu/media/sfylifasufledu/baker/docs/pdf/horticulture/demonstration-gardens/Passionflower-Vine-and-Florida-Butterflies.pdf

The sweet granadila is the common form of passiflora and its care is somewhat different. It doesn't like very hot weather, but according to your average temperatures, Nairobi doesn't have hot weather. I wonder if it is getting enough water. There is some kind of environmental stress or there is some type of bud mite affecting the blooms. This article gives tips on care. Compare those to yours:

https://hort.purdue.edu/newcrop/morton/sweet_granadilla.html

If your passion flower produced fruit that was left on the vine, it is also reproducing by seed. I have found the vines pull up easily, but as you say, there could be bits left behind to regrow. You can spray with glyphosate, 2,4D and others as noted in this article. Read and follow all instructions exactly.

https://www.gardeningknowhow.com/ornamental/vines/passion-flower/maypop-weed-control.htm