Adjust the Duration Type for new tasks to be something other than Calculated Assignment. You can make this adjustment system-wide, or you can make this adjustment for a specific group(s). If the converted task needs to be Calculated Assignment, you can change the Duration Type of the task after it has been created.
Configure system-wide task and issue preferences
Adjust the Default Duration for issues in the request queue. If you navigate to the project that you have designated as a request queue (where people are submitting issues and requests), you can dictate the default duration of an issue. After navigating to the request queue project, go to Queue Details > Default Duration. If this is set to 0, then the Planned Hours of issues will also be set to 0, so there won't be any Planned Hours that carry over during the task conversion.
Create a Request Queue
