Hello again,
If the issue isn’t a pest, it may be a nutrient deficiency. The way you describe the issue, I think the issue may be a micro nutrient deficiency of magnesium or possibly iron. Earth Juice Microblast is a product that I think may help.
When my plants begin to have problems out of the blue, I try to think if I have changed any of the conditions recently. This can range from a change or lapse in watering, nutrient feeding, repotting, exposure to cooler temperatures, exposure to a new plant (pest transfer), or moving the plants location. If this is the case, try to baby the plant while it recovers by being diligent in watering when appropriate, and giving the plant a break from the normal feeding schedule.
Good luck!