The symptoms are being caused by root damage. This is an example of what can happen when a plant is repotted unnecessarily. ZZ Plants especially need to be kept quite potbound in order to thrive. Without knowing the details of your repotting, I can only suggest a few possibilities of what is causing the problem with the root system.
If you removed some or all of the original soil, then many of the tiny roothairs were probably damaged inadvertently. Those roothairs, that are barely noticeable, do most of the work for the plant.
The new soil added should have been very porous with extra perlite added so that it dries out more quickly. The added soil acts like a sponge and absorbs and retains water for a long time. ZZ Plants are tough plants, but the one thing they do not tolerate is excess moisture around their roots. They can withstand drought, but not constant dampness. Rotted roots cannot absorb water for the rest of the plant and the result is wilted leaves that makes it appear that it is under-watered.
If you added new soil to the surface of the original rootball, it is preventing the soil in the root zone from drying out quickly enough. If so, it should be removed.
It appears that the planter it is potted in has no drain holes. The common practice of adding "drainage material" to the bottom of the pot is not an effective substitute for using pots with drain holes. Without drain holes, you have no way to tell how much water to add. If you add just a little too much each time you water, eventually the water accumulates in the bottom of the pot and that is deadly for a plant like a ZZ.
So, a lot can easily go wrong when repotting is done, even with the best of intentions. Repairing the damage is not so easy. You could undo the repotting, by removing the soil you added and moving the original intact rootball back into its original pot. However, that disturbance may cause further trauma.
A better alternative is to allow the soil to dry at least halfway deep into the pot before adding any water. This will be hard to determine and may require a wooden chopstick used as a soil probe to make that determination. When it reaches that level of dryness, add just enough water so that it reaches that level of dryness again in about a week. Experiment to find out what the right amount of water is.
In the interim, keep the plant in good light and warm temps. If you have caught the problem soon enough, you may see a gradual recovery, but you will have to be patient. Healthy new growth is the best indicator of recovery. Older leaves and stems are unlikely to recover fully.
