1. What kops version are you running? The command kops version, will display
this information.
Version 1.18.1 (git-453d7d96be)

2. What Kubernetes version are you running? kubectl version will print the
version if a cluster is running or provide the Kubernetes version specified as
a kops flag.
v1.19.3
3. What cloud provider are you using?
aws
4. What commands did you run? What is the simplest way to reproduce this issue?
kops create cluster --help and I've run:
export NODE_SIZE=${NODE_SIZE:-m4.large}
export MASTER_SIZE=${MASTER_SIZE:-m4.large}
export ZONES=${ZONES:-"us-east-1d,us-east-1b,us-east-1c"}

export NODE_SIZE=${NODE_SIZE:-m4.large}
echo $NODE_SIZE
m4.large

then I've tried change data:

export NODE_SIZE=${NODE_SIZE:-t3.small}
echo $NODE_SIZE
m4.large

5. What happened after the commands executed?
variables in export have had constant values

6. What did you expect to happen?
I've got variables without redefine values.
I propose to use classic syntax :
export ZONES="us-east-1d,us-east-1b,us-east-1c" instead of the export ZONES=${ZONES:-"us-east-1d,us-east-1b,us-east-1c"}

and change information in official docs about non-use such construction.