I've had this ZZ plant for a couple of years, and it has been great, no problems. But I'v,e noticed that just one part of my ZZ plant is yellow and the leaves on another stem are curling in (can't recall proper terms but you get the idea). The rest of the plant looks fine. It isn't near a window, heat source etc. In same place at it has always been.

I have another zz that is just a few feet away, and it has curling on just one stem.

Any thoughts would be most appreciated. Many thanks.

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

As Gina pointed to, one leaf is just dying. No big deal. All leaves have a finite lifespan.
I do see that the rachis (leaf stems) show wrinkles; this is due to extreme water stress. One does have to water ZZ plants. As all the leaf stems are showing the wrinkles, I don't think it is a case of rot, but drought.
I would put it in a container and give a good 10 min. soak. See if that doesn't improve its appearance in a couple of days.
If it doesn't, unpot it and check for rotten roots.