When you use a Write Automation command on an automation playlist with no previously written automation data, the selected value is written to the entire playlist and not just the selected area.
This is because a playlist with no automation data contains only a single automation breakpoint that corresponds to the current position of the control for the parameter. The position of the breakpoint is updated whenever the parameter value is changed.
If you do not want the Write Automation command to write the selected automation value to the entire playlist, you can:
This lets the Write Automation command write only to the selected area.