Dragging the line of the Planning Console's Gantt chart causes the Project Task to become a Milestone.
This was happening due to the customization of the Planning Console Column for Actual Duration.
A custom field was being used rather than the OOB one for "work_duration".
Once this was changed back to "work_duration", everything worked as expected.
1. Navigate to "pm_console_column.LIST"
2. Find the column for Actual Duration and go to the record for the "pm_project_task" Console Table
Make sure to click the " i " button and then "Open Record"!
3. In the Column field, select "Actual duration(work_duration)"
4. Save the record and test moving the Gantt Chart line in the Planning Console