Can you post a picture of your plant and tell us about the growing conditions? Is it inside or out? And if outside what geographical location?

I live in singapore, tropics very near the equator, hot and humidity weather all year round. my potting medium is 311, pine/perlite/peatmoss. Got the seeds from the USA, its called blue spice basil. I sowed the seeds about 5 months back and have pruned it once. And the plant is on pro-tekt and FP936 dosed daily according to manyfacturer's instructions. It almost feels like the plant is having a balding situation.

this other basil called pistou basil, seeds from the same nursery is also beginning to have same issue. The leaves that off are the healthy green ones.

even the new small leaves are dropping off

I have never experienced this issue with my basil. I previously grew sweet basil but it did not have this problem. what could be causing this shedding of healthy green leaves?

the location of the plant is outdoors, high rise apartment. shaded but v bright receiving 3-4 hours of direct sun daily.