Disturbing roots when replanting or dividing is always dicey unless you are experienced. It is very easy to damage delicate root hairs when disturbing roots. Using proper pot size and potting mixes is also important and often misunderstood.
ZZ Plants are very drought resistant and thrive when kept very potbound. Inadvertent over watering is one of the few things they don't tolerate. Using a pot that is too large and filled with excess soil that is too dense often leads to root rot. Now that you have divided and repotted be very careful not to over water.
