Indoor ZZ Plant with One Leaf Turning Yellow

My ZZ Plants are extremely slow growing, but require next to no attention at all. I'll go weeks without thinking twice about them, and then remember that they are alive and need water. I divided one clump that I bought into 2 pots, and put one in the bathroom and one in the front office. The bathroom only has a tiny 3 ft x 6 in window, so I'm amazed these plants are able to thrive and put on new growth in such dark dry conditions.

Only the 2nd new sprout on this plant in about 11 months!

The one in the front office is putting up a new shoot. This is always exciting because it's only happened 4 times in the year that I've owned them. They are succulents and store energy in a massive bulb under the soil - then when they build up enough resources to launch a new sprout it shoots up relatively quickly.

Slow growing ZZ plant with yellow leaf on the very oldest stem

I have 1 conspicuous yellow leaf on the plant in the office. It looks a little bizarre. It's from the oldest sprout on the plant, so probably just letting it go to favor the new growth. It isn't a blotchy yellow like some photos I've seen online, it's a total yellowing. I hear these things sometimes lose leaves seasonally as well, but this is my first one.