Q.Why Is It Happening? And How Can I Fix It?
This was a cutting and rooted nicely in water. there were more leaves maybe 4-6 this is the last one and this is how it looks
Certified GKH Gardening Expert
Some plants rooted in water have trouble acclimating to soil. It looks like a new leaf is sprouting, so just let the other one drop. Make sure you aren't overwatering. The pot may be too large for the plant and the excess soil is retaining too much moisture. I suggest moving it to a smaller container with fresh soil. Only water when the top feels dry.
Here is more:
https://www.gardeningknowhow.com/houseplants/dumb-cane/growing-dumbcane-dieffenbachia.htm