Default description behaviour overwrites edited description if issue type changed, after default description is edited, but prior to the "Create" button being used to create the issue.
The problem cause JIRA to popup a warning message:
These fields have default values for this project and issue type combination. We have preserved the existing values in case you need them
Modify your initialiser scripts as shown here - https://scriptrunner.adaptavist.com/latest/jira/behaviours-use-cases.html#_setting_a_default_description
Only set the default value if the current form has the empty field, rather than the current issue.