Is the leaflet in question on the bottom? If so, I have read that a small amount of lower leaves turning yellow and falling is considered normal.
However, yellowing leaves on a ZZ plant can be attributed to too much light or too much water. I would review and apply the knowledge found in the following article, which discusses the care of ZZ plants: https://www.gardeningknowhow.com/houseplants/zz-plant/caring-for-zz-plant.htm
By the way - you can cut away that leaflet that is turning yellow near the base of a stem.