I have a couple zz plants I find one way to tell when underwatering is the is if its dry and the bottom leaves get pale and yellow and die like especially one leaf parts that stick straight up out of the soil or rhyzome, especially baby ones. Curling could be big temperature change?
Come to think of it though I love these plants cause I can put them near hvac vents with no probs lol so its probably from overwatering, not enough light, or repotting if you didn't see no leaves turn yellow,
