Automation help - auto fill date when task status = available

Hi there, 

I think this is a simple automation but I was stuck here for a while. 

I hope to set up an automation to update the "start date" of a task to "current date" when the task status is changed to "available".  I was able to set up the trigger (when "status" is changed and is equal to "available"), and a action ("start date" copy the value from the field "Current Date". ) 

While this automation successfully updates the "start date" of a task to "current date" after I changed the status to "available", it updates something I don't want to --  it updates "start date" of other existing available tasks. 

Result:
Task 1 is changed to "available" on 4/10, and the "start date" is 4/10. 
Task 2 is changed to "available" on 4/15, and the "start date" is 4/15. In addition, it updates Task 1's start date to 4/15 too. The second part is not what I want. 

What did I missed here? Thank you very much. 

Ning