When you say the roots looked normal, what did they look like? What about the rhizomes? How often does your ZZ get watered? Does that pot it is in have drainage?

To me it looks like they rotted at the base which indicates too much water, but more info would help.

Do you have a cat?

My guess is the soil probably not having the correct nutrients/pH. Other suspect is over watering or not enough drainage, but that's not likely IMO. Maybe repot with adding 50% gardening soil and water sparingly with some rainwater and add some low intensity fertilizer - it should come back with new sprouts.

Best way to propagate is to cut a piece off the root ball with stem and all, as this develops much faster.

Just the stem works too in water, but takes much longer. You leave it in the water and change it regularly until it grows about an inch or two of roots. Then plant it in some proper soil (mix of coco choir and soil worked great for me) and wait about 6 months, watering sparingly, as the root ball develops. Only then will the cutting slowly die off and new leaves will begin to sprout out of the soil. You'll have to be patient though.

Don't forget to use fertilizer every few weeks from spring to fall, but a gentle one, I always use standard vegetable fertilizer in a medium dose

zap zap plant