Certified GKH Gardening Expert
This appears to have two causes. The plant is getting leggy in response to a lack of light. Even though it can handle lower light, there is a lower limit. This will need just a little more to be happy.
The browning could be something a little more sinister, though.
This is often caused from fungal infections, caused by overwatering. This will need to be fixed with a fungicide. This article will help: https://www.gardeningknowhow.com/garden-how-to/info/using-fungicides-in-garden.htm
This article will help you with the care of ZZ plants: https://www.gardeningknowhow.com/houseplants/zz-plant/caring-for-zz-plant.htm