I live in the PNW near Seattle, but because it is summertime, nighttime temperatures are consistently above 10 degrees celsius and daytime temperatures are around 27.
It started looking like this several months ago when it prepared to flower and it was still indoors for the winter. I put it outside and the condition kept deteriorating. The fruit also seems to not be getting any bigger.
About nine months ago when I planted it I filled the pot with potting soil and about two months ago my mum covered it with sand to keep water in or something. She raises a lot of houseplants so I assumed she was experienced on this matter.
I water it about every three days with about 250 ml water and a few drops of MiracleGro (it also occasionally rains). I read that to help iron deficiencies people put rusty nails into the soil so I stuck some old screws into the soil.
Is it the sand? I have a sneaking suspicion it is the sand.
That fruit looks great for a tropical plant growing way up north!
The issue is not the sand. Top dressing with sand, mulch, rocks, etc is a common practice with potted plants that at worst does no harm.
My guess is over watering is getting to it. Try just thoroughly soaking it (as in watering until some starts coming out the bottom of the pot) ~ once a week. The idea is to let the soil dry out a little between waterings. You can always stick your finger into the soil to see if it is still damp to see if watering is needed.