I am not an expert on ZZ plants even though I have 5. But...it looks like that stem is dying. ZZ plants are weird aroids, in the same group of plants somewhat as Anthuriums and Philodendrons and Monsteras...all are aroids but ZZ plants are in a really different part of that group. What we would think about as a leaf (the small structures coming off the stem) is not the leaf...the entire stem is one leaf that arises from rhizomes under the ground. It's possible that under the soil, those leaves in the area that are dying back may be overwatered. When you water do you pour the water in in the same place every time? That rhizome may be getting too much and rotting. ZZ's are more like succulents. They need very little water.
Maybe others will chime in
