This issue is a pita if you encounter it and need to deploy additional templates.
There is a workaround if you already have a feature template, made with pre 19.2.1 , and have the DHCP Helper as a variable, then you can edit the variable when you attach devices to the template. You can also reuse that old template copying it modifying the required parameters, as long as the dhcp helper was a variable from its creation with a earlier vManage.
The statement
"Workaround:
Configuration can be still done using CLI"
Is not possible with vManaged cEdges using featuretemplates.